School‐based programmes for preventing smoking
Abstract
Background
Helping young people to avoid starting smoking is a widely endorsed public health goal, and schools provide a route to communicate with nearly all young people. School‐based interventions have been delivered for close to 40 years.
Objectives
The primary aim of this review was to determine whether school smoking interventions prevent youth from starting smoking. Our secondary objective was to determine which interventions were most effective. This included evaluating the effects of theoretical approaches; additional booster sessions; programme deliverers; gender effects; and multifocal interventions versus those focused solely on smoking.
Search methods
We searched the Cochrane Central Register of Controlled Trials (CENTRAL), the Cochrane Tobacco Addiction Group's Specialised Register, MEDLINE, EMBASE, PsyclNFO, ERIC, CINAHL, Health Star, and Dissertation Abstracts for terms relating to school‐based smoking cessation programmes. In addition, we screened the bibliographies of articles and ran individual MEDLINE searches for 133 authors who had undertaken randomised controlled trials in this area. The most recent searches were conducted in October 2012.
Selection criteria
We selected randomised controlled trials (RCTs) where students, classes, schools, or school districts were randomised to intervention arm(s) versus a control group, and followed for at least six months. Participants had to be youth (aged 5 to 18). Interventions could be any curricula used in a school setting to deter tobacco use, and outcome measures could be never smoking, frequency of smoking, number of cigarettes smoked, or smoking indices.
Data collection and analysis
Two reviewers independently assessed studies for inclusion, extracted data and assessed risk of bias. Based on the type of outcome, we placed studies into three groups for analysis: Pure Prevention cohorts (Group 1), Change in Smoking Behaviour over time (Group 2) and Point Prevalence of Smoking (Group 3).
Main results
One hundred and thirty‐four studies involving 428,293 participants met the inclusion criteria. Some studies provided data for more than one group.
Pure Prevention cohorts (Group 1) included 49 studies (N = 142,447). Pooled results at follow‐up at one year or less found no overall effect of intervention curricula versus control (odds ratio (OR) 0.94, 95% confidence interval (CI) 0.85 to 1.05). In a subgroup analysis, the combined social competence and social influences curricula (six RCTs) showed a statistically significant effect in preventing the onset of smoking (OR 0.49, 95% CI 0.28 to 0.87; seven arms); whereas significant effects were not detected in programmes involving information only (OR 0.12, 95% CI 0.00 to 14.87; one study), social influences only (OR 1.00, 95% CI 0.88 to 1.13; 25 studies), or multimodal interventions (OR 0.89, 95% CI 0.73 to 1.08; five studies). In contrast, pooled results at longest follow‐up showed an overall significant effect favouring the intervention (OR 0.88, 95% CI 0.82 to 0.96). Subgroup analyses detected significant effects in programmes with social competence curricula (OR 0.52, 95% CI 0.30 to 0.88), and the combined social competence and social influences curricula (OR 0.50, 95% CI 0.28 to 0.87), but not in those programmes with information only, social influence only, and multimodal programmes.
Change in Smoking Behaviour over time (Group 2) included 15 studies (N = 45,555). At one year or less there was a small but statistically significant effect favouring controls (standardised mean difference (SMD) 0.04, 95% CI 0.02 to 0.06). For follow‐up longer than one year there was a statistically nonsignificant effect (SMD 0.02, 95% CI ‐0.00 to 0.02).
Twenty‐five studies reported data on the Point Prevalence of Smoking (Group 3), though heterogeneity in this group was too high for data to be pooled.
We were unable to analyse data for 49 studies (N = 152,544).
Subgroup analyses (Pure Prevention cohorts only) demonstrated that at longest follow‐up for all curricula combined, there was a significant effect favouring adult presenters (OR 0.88, 95% CI 0.81 to 0.96). There were no differences between tobacco‐only and multifocal interventions. For curricula with booster sessions there was a significant effect only for combined social competence and social influences interventions with follow‐up of one year or less (OR 0.50, 95% CI 0.26 to 0.96) and at longest follow‐up (OR 0.51, 95% CI 0.27 to 0.96). Limited data on gender differences suggested no overall effect, although one study found an effect of multimodal intervention at one year for male students. Sensitivity analyses for Pure Prevention cohorts and Change in Smoking Behaviour over time outcomes suggested that neither selection nor attrition bias affected the results.
Authors' conclusions
Pure Prevention cohorts showed a significant effect at longest follow‐up, with an average 12% reduction in starting smoking compared to the control groups. However, no overall effect was detected at one year or less. The combined social competence and social influences interventions showed a significant effect at one year and at longest follow‐up. Studies that deployed a social influences programme showed no overall effect at any time point; multimodal interventions and those with an information‐only approach were similarly ineffective.
Studies reporting Change in Smoking Behaviour over time did not show an overall effect, but at an intervention level there were positive findings for social competence and combined social competence and social influences interventions.
Plain language summary
Can programmes delivered in school prevent young people from starting to smoke?
Increasing numbers of young people are smoking in developing and poorer countries. Programmes to prevent them starting to smoke have been delivered in schools over the past 40 years. We wanted to find out if they are effective.
We identified 49 randomised controlled trials (over 140,000 school children) of interventions aiming to prevent children who had never smoked from becoming smokers. At longer than one year, there was a significant effect of the interventions in preventing young people from starting smoking. Programmes that used a social competence approach and those that combined a social competence with a social influence approach were found to be more effective than other programmes. However, at one year or less there was no overall effect, except for programmes which taught young people to be socially competent and to resist social influences.
A smaller group of trials reported on the smoking status of all people in the class, whether or not they smoked at the start of the study. In these trials with follow‐up of one year or less there was an overall small but significant effect favouring the controls. This continued after a year; for trials with follow‐up longer than one year, those in the intervention groups smoked more than those in the control groups.
When trials at low risk of bias from randomisation, or from losing participants, were examined separately, the conclusions remained the same. Programmes led by adults may be more effective than those led by young people. There is no evidence that delivering extra sessions makes the intervention more effective.
Authors' conclusions
Background
Children and adolescents in all cultures smoke, with increasing rates in many developing countries. Starting smoking usually leads to the behaviour lasting decades, with great difficulty in quitting. Few studies verify smoking by biochemical tests, and self reported rates probably underestimate true rates. Smoking uptake is associated with existing smoking by family and friends, and with risk‐taking behaviours. Researchers have implemented programmes to counteract these influences. Programmes in schools have evolved over four decades and include those providing information about smoking rates and harms from smoking; teaching children how to be more socially competent to avoid starting smoking; teaching skills to refuse offered tobacco and multimodal programmes with parents, teachers, and the community.
The incidence and prevalence of smoking among children and adolescents
Tobacco use is the main preventable cause of death and disease worldwide, and the five million deaths annually attributable to tobacco use are predicted to increase to eight million annually by 2030 (Warren 2009). Of the US population who were 17 or younger in 1995, it was estimated that five million would die prematurely of tobacco‐related causes, and that 20% of deaths could be avoided if smokers had either never started or had quit (Epstein 2000b).
The World Health Organization (WHO) 'Health behaviour in school‐aged children 1997‐8' survey of 11, 13 and 15 year olds in 29 countries (Europe, Canada and the USA) found that for the 15 year olds in 14 countries more than 20% of females, and in 11 countries more than 20% of males smoked daily (WHO 2000). Surveys of the smoking behaviour of 13 to 15 year olds were then conducted between 1998 and 2008 in all six WHO world regions with 100 initial, 100 second and nine third surveys involving 530,849 students. In 191 of the 209 surveys, more than 90% of the schools participated, and in 190 of 209 surveys, student participation was greater than 80%. The prevalence of both cigarette smoking and other forms of smoking such as water pipes, were both defined as at least monthly (Warren 2009).
For the 100 sites with follow‐up surveys, there were increases in the prevalence of smoking cigarettes at least one day per month at 27 sites and decreases at 10, and for other tobacco products (such as water pipes) at least one day per month there were increases at 33 sites and decreases at 13 (Warren 2009). Therefore, if poorer countries follow the trajectory of the more affluent countries, it is to be expected that 20% to 30% of 13 to 15 year olds may smoke, depending on the culture of the country and the activities of the tobacco companies.
Adolescent smoking remains a risk factor in adulthood. The 1995 US National College Health Risk Behavior Survey found that 70% had ever tried smoking a cigarette, and of these 42% were current smokers and 13% current daily smokers. Females were more likely to smoke than males (Pletcher 2000). Adolescents who begin smoking at younger ages are more likely to become regular smokers and less likely to quit (Tyas 1998). Of concern is the finding that the first use of tobacco after age 18 in the USA increased from 25% to 40% between 2002 and 2009 (SAMHSA 2009).
Villanti 2010 identified five types of smoking behaviour as adolescents become young adults: nonsmokers, early stable smokers, late starters, quitters, and 'light or intermittent smokers'. In adulthood, the early stable and late starter groups had the highest rates of smoking, but the light or intermittent smokers could go either way, and after two years had either temporarily quit or had become heavy smokers.
School‐based interventions
Over the past three decades the school environment has been a particular focus of efforts to influence youth smoking behaviour. The main perceived advantages are that almost all children can be reached through schools, and a focus on education fits naturally with the daily activities of schools. Researchers have used five types of interventions in schools, each based on a different theoretical orientation:
1. Information only curricula
Interventions that provide information to oppose tobacco use (also called normative education) are described by Griffin 2010 as "content and activities to correct inaccurate perceptions regarding the high prevalence of substance use." Griffin describes how many adolescents overestimate smoking prevalence and view smoking as normative behaviour. Normative curricula seek to inform students on actual rates of use and undermine inaccurate beliefs on the social acceptability of smoking. Normative materials are often used by programme deliverers in social resistance programmes. The assumption is that information alone will lead to changes in behaviour (Bangert‐Drowns 1988).
2. Social competence curricula
A group of interventions that aim to help adolescents refuse offers to smoke by improving their general social competence. Griffin 2010 recognises that poor personal and social skills can lead to development of drug use. Therefore, programmes benefit from including social learning processes or life skills such as problem‐solving and decision‐making, cognitive skills for resisting interpersonal or media influences, increased self control and self esteem, coping strategies for stress, and general social and assertive skills. These skills will also have broader applications for the students. The interventions are based on Bandura's social learning theory (Bandura 1977), which hypothesises that children learn drug use by modelling, imitation, and reinforcement, influenced by the child's pro‐drug cognitions, attitudes and skills. Susceptibility is increased by poor personal and social skills and a poor personal self concept (Botvin 2000).
3. Social influence curricula
Interventions that aim to overcome social influences promoting tobacco use by providing skills to adolescents (also called social skills interventions). Griffin 2010 describes these interventions as aiming to increase the "adolescents’ awareness of the various social influences that support substance use." Programmes adopt resistance skills training in which students are taught how to deal with peer pressure, high risk situations, how to effectively refuse attempts to persuade substance use from both direct and indirect sources. The interventions are based on McGuire's persuasive communications theory and Evans's theory of psychological inoculation (McGuire 1968; Evans 1976).
4. Combined social competence and social influences curricula
Methods that draw on both social competence and social influence approaches.
5. Multimodal programmes
These programmes combine curricular approaches with wider initiatives within and beyond the school, including programmes for parents, schools, or communities and initiatives to change school policies about tobacco, or state policies about the taxation, sale, availability and use of tobacco.
Why it is important to do this review
Tobacco education curricula are widely used in US schools, though few of those in use have been rigorously evaluated. The US 2000 National Youth Tobacco Survey national sample of 35,828 6th‐ to12th‐graders in 324 schools found that 70% of the middle schoolers and 50% of the high schoolers said they had received a programme that taught them the short‐term consequences of tobacco use. The percentages for receiving a normative programme were 40% and 18%; for programmes teaching why people smoke 64% and 38%; for programmes teaching refusal skills 51% and 17%; and for multi‐strategy programmes 38% and 17% (Wenter 2002). Wiehe 2005 identified eight programmes that followed participants to age 18 or the 12th grade and found little or no evidence of effectiveness. There is nevertheless continued uncertainty about both the relative and absolute effectiveness of school‐based programmes, and considerable variation in the extent to which they are implemented in other countries.
This review is important because there is no other systematic review of world literature on school‐based smoking prevention programmes without language or date restrictions. This review was first published in 2002. This update has refined how the included studies are categorised to provide analysis based on Pure Prevention cohort studies, Change in Smoking Behaviour over time studies and Point Prevalence of Smoking studies.
Objectives
The primary objective of this review is to assess the effectiveness of school‐based programmes in preventing children and adolescents from starting smoking. A secondary objective is to assess which programme elements, if any, are associated with effectiveness.
We considered one central question:
Are school programmes, categorised by intervention type, more effective than minimal or no intervention in preventing smoking? We considered the hypothesis that they are more effective separately according to the theoretical orientation of the prevention programme:
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Information giving
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Social competence
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Social influence
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Combined social influence and social competence
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Multimodal programmes
If the review showed the effectiveness of one or more of these types of intervention, we proceeded to the secondary objective, i.e. to examine the direct evidence comparing different types of intervention, categorised by theoretical orientation, including:
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Social influences versus information giving
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Social influences versus social competence
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Combinations of social influences, social competence and information versus single component interventions
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Multimodal programmes versus single component interventions
We also aimed to consider the effect by gender and the method of programme delivery, including:
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Peer‐led programmes versus those taught by researchers or teachers
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Booster sessions after programme completion versus no booster
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Tobacco‐focused interventions versus interventions focused on tobacco together with other substances such as alcohol and drugs
Methods
Criteria for considering studies for this review
Types of studies
We included studies in which individual students, classes, schools, or school districts were randomised to receive different programmes or to be the control, and in which baseline tobacco use was measured. We excluded studies if they did not state that allocation of individuals or groups to intervention and control groups was randomised. Random allocation of intervention was either to the individual or to individuals in clusters (in classes, in schools, in classes nested within schools, or in school districts). We assessed whether the studies were analysed using methods appropriate to the level of allocation and the level of measurement of the outcomes. No studies were excluded on the basis of publication status or language of publication.
Types of participants
Children (aged 5 to 12) and adolescents (aged 13 to 18) in school settings. We also included studies in which the participants were 5 to 18 during the intervention phase of the study, but were followed up in a few instances beyond 18.
Types of interventions
We included all school‐based programmes that had as one of their goals preventing tobacco use, irrespective of theoretical intervention. Some programmes aimed simply to provide information about tobacco. Others had more complex goals: teaching generic social skills to reinforce societal norms about individual behaviour; reinforcing the adolescent's self concept; and teaching social skills and specific tobacco refusal skills. Some focused on multiple addictions, and we included any programmes with any drug or alcohol focus provided outcomes for tobacco use were reported. Some focused on 'healthy schools.' We included these provided outcomes for tobacco use were reported. We classified programmes according to their dominant theoretical orientation and then allocated them to one of the five categories described in the Background section or to a sixth category, 'other'. Programmes that solely provided information were placed in the information only category, while recognising that all curricula provided information to participants.
For each study we determined whether the intervention programmes were compared with a control group, and whether the control group received no intervention, or the standard health education curriculum taught in the school, or the tobacco education curriculum in normal use in the school.
There were no restrictions on who delivered the intervention. These could include researchers, classroom teachers, health science teachers, healthcare professionals, undergraduate or graduate students, adolescent peers, or other personnel.
Types of outcome measures
The primary outcome was the effect of the intervention on the smoking status of individuals or cohorts who reported no use of tobacco at baseline. We recorded whether effects of the interventions were found at the conclusion of the programme, and whether such effects were sustained at follow‐up after completion of the programme. We required a minimum follow‐up of six months after the intervention.
We did not require biochemical validation (by saliva thiocyanate or cotinine or expired air carbon monoxide levels) of self reported tobacco use for inclusion, but recorded its use. If saliva samples were collected but not analysed (sometimes described as the 'bogus pipeline' procedure), this was recorded.
One problem in this field is that the studies often use different measures of tobacco use, either recording frequency (monthly, weekly, daily), or the number of cigarettes smoked, or an index constructed from multiple measures. Sometimes the variety of measures is intended to record the fact that young children begin smoking on a monthly basis, but as they get older may proceed to weekly and daily smoking. We excluded studies which did not report any measure of smoking behaviour, studies that did not assess baseline smoking status in the pre‐test survey, and studies that reported only changes in knowledge or attitudes about smoking.
Search methods for identification of studies
We searched the following databases using search strategies similar to those used in MEDLINE for each. Detailed search strategies are displayed in Appendix 1 (MEDLINE) and Appendix 2 (CINAHL):
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MEDLINE 1966 ‐ 10/2012
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EMBASE 1974 ‐ 10/2012
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CINAHL ‐ 10/2012
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PsycINFO 1967 ‐ 10/2005
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ERIC 1982 ‐ 10/2005
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Health Star
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Tobacco Control 1992 ‐ 2005
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Journal of Smoking Related Disorders 1990 ‐ 2005
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Dissertation Abstracts 1960 ‐ [Search strategy = (Tobacco or smoking) and prevent? and (child or adolescent)]
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US Department of Health Reviews
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Proceedings of the World Conferences on Tobacco and Health
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Cochrane Tobacco Addiction Review Group Specialised Register 10/2012
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Reference lists of the articles selected in the above sources
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Index of Scientific and Technical Proceedings
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Conference Papers Index
In addition, we searched MEDLINE from 1966 to October 2012 for 133 individual authors who had published in the field. We also screened the reference lists of the included studies.
None of the previous meta‐analyses of the literature (listed in the additional references below) undertook a Cochrane search strategy.
The most recent searches were conducted in October 2012.
Data collection and analysis
Selection of studies
Two authors (RET and JM) independently assessed the search results for studies that met the inclusion criteria. Reference lists were checked for further relevant studies. The full text of each study was independently assessed, and the authors contacted for clarification in cases of uncertainty.
Data extraction and management
Two authors (RET and JM) independently extracted data, with disagreements resolved by recourse to co‐author RP. We categorised studies into six groups corresponding to the type of intervention (information; social competence; social influence; combined social competence and social influence; multi‐modal, and other). Information extracted included country of study, intervention focus, description of participants (numbers of participants, classes and schools, age, gender, ethnicity, existing smoking status), description of intervention (duration, nature, deliverer, outcome, follow‐up), quality of delivery, and statistical methods.
Assessment of risk of bias in included studies
Two authors independently assessed five aspects of risk of bias, with adjudication in case of disagreement by a third author. Each potential risk of bias was assessed to be either at low or at unclear risk (if no data were provided which could be judged to assess bias), or at high risk (study design or execution could cause over‐ or underestimation of the intervention effect). We contacted authors to verify any risk of bias information not presented in their publications.
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Sequence generation (selection bias)
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Allocation concealment (selection bias)
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Blinding of outcome assessment (detection bias), which was assessed as unclear unless a specific reference was made to blinding of outcome assessors.
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Incomplete outcome data (attrition bias) due to absence of some data for individuals or loss of all data for an individual after a certain time. We examined studies for systematic differences in the rate of loss to follow‐up among different groups. Where there was differential attrition between groups, we considered bias was more likely if there was no sensitivity analysis of the effect of this attrition on outcomes.
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Selective reporting (reporting bias) due to authors either (1) not reporting all outcomes as determined by the objectives stated initially in their study protocol, or previous publications about the study or within the current publication, or (2) reporting only a subset of outcomes with significant results.
Data synthesis
We identified three groups of studies:
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Pure Prevention cohorts (Group 1): Cohorts in which never‐smokers at baseline were followed and the number remaining never‐smokers at the various follow‐up intervals was ascertained. Where authors did not report these data we either computed them from the published articles or we contacted authors and requested that they compute these data. We obtained absolute numbers or odds ratios from individual randomised trials with the control group as comparator. Where the authors used a denominator which did not include all the participants originally randomised (e.g. a sample which the author described as the 'analysis sample,' which excluded drop‐outs and thus had smaller numbers at follow‐up), we recomputed the data using the numbers originally randomised. We calculated adjusted odds ratios based on the number of never‐smokers at specific time points. Adjustment was made for clustering by school/group based on either reported or estimated intraclass correlation coefficients (ICCs) and cluster sizes to determine design effects for each of the intervention groups. We then used this design effect to determine the effective sample size for each intervention group. We obtained a pooled estimate of the effect using the generalised inverse variance method and a fixed‐effect model. We conducted subanalyses for Group 1 based on gender, peer‐led (or substantially peer‐led) versus adult‐led studies, tobacco as the sole focus of the intervention versus multifocal interventions, and interventions that had subsequent booster sessions versus those with none.
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Change in Smoking Behaviour over time (Group 2): Studies where the smoking behaviour was measured as change over time. These studies included those with growth curve analysis. We extracted summary measures for the change in smoking status/use from each study in this group. These were reported either for each study group (mean change or ß‐coefficient of change over time plus their associated standard error by study arm) or as an overall change measure attributed to the intervention (odds ratio (OR) with 95% confidence interval (CI), ß‐coefficients of linear change and associated standard error; one per study comparison). When overall effects were reported as ORs and 95% CI we transformed these into standard mean difference (SMD) by multiplying by √3/∏ = 0.5513 as recommended in the Cochrane Handbook (9.4.6 Combining dichotomous and continuous outcomes).
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Point Prevalence of Smoking (Group 3): Studies reporting smoking prevalence at baseline and follow‐ups. Individuals were not followed individually to the follow‐up points, and thus the prevalence rates at baseline and follow‐up are cross‐sectional data. Measures included mean usage (indices and ever‐use), percentage in the past week, past month, lifetime usage, percentage smoker and percentage never‐ or nonsmoker. We calculated a summary measure by comparing the difference in smoking prevalence from baseline to follow‐up between the two arms. We obtained the standard error by estimating the correlation of smoking status from data available from a small selection of Group 1 studies, and using the total number of clusters as a proxy for sample size in each group.
For both Change in Smoking Behaviour over time (Group 2) and Point Prevalence of Smoking (Group 3) studies:
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We extracted the most conservative smoking outcome, i.e. the lowest usage of smoking (ever‐smoked, and if not available then monthly smoking).
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We contacted all authors (after 1995), asking them to identify a cohort of baseline never‐smokers, which would allow the study to be included in the Pure Prevention cohorts (Group 1)
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We obtained a pooled estimate of the effects using the generalised inverse variance method and a standardised mean difference.
If a study provided data that were applicable to more than one group of studies, then the data were accepted for both groups. Data from all three groups were not pooled, but were analysed separately throughout the review.
The three groups (Pure Prevention cohort, Change in Smoking Behaviour over time, Point Prevalence of Smoking) were each analysed as an entire group, and then by the intervention used (information; social competence; social influences; combined social competence and social influences; multimodal; other interventions). Studies in the 'other interventions' group were sufficiently different from each other that, although they were presented within the meta‐analysis for the entire group, it would not be appropriate to combine them as a subgroup by intervention within the Results and Discussion sections.
For all groups, study results were analysed by outcomes of one year or less, and then by longest available follow‐up point. The raw data are tabulated in Appendix 3; Appendix 4; Appendix 5.
Where a study compared more than one intervention arm the control group was split equally between them for both outcome events and sample size. The additional intervention arms within the study were added to the review with a text link to the first.
All RCTs were cluster‐randomised trials (C‐RCTs), except for one trial (Werch 2005), and calculations to allow for the effects of clustering using intraclass correlation coefficients (ICCs) were either made by the study authors or were applied by the review authors.
All studies included in the review were assessed and placed into one of the three analysis groups above (Pure Prevention cohorts, Change in Smoking Behaviour over time, and Point Prevalence of Smoking). Studies were included in the review but excluded from the analysis if, once allocated to one of the three analysis groups, it was established that data were missing from studies, such as no baseline and follow‐up numbers, no control arm data, or the review authors were unable to reconcile the data. In these instances we contacted the study author. If there was no response or data were no longer available for these studies then it was not possible to include the studies in the analysis. In some instances if data were available, but only the total number of schools or classes was known and not the numbers allocated to each arm, then the number of schools or classes was estimated based on the proportion of individuals within the group.
Results are presented as: descriptive text, tables and forest plots (pooled data).
Subgroup analysis and investigation of heterogeneity
We used the I² statistics to assess inconsistency across studies and provide a measure of heterogeneity (Higgins 2003). Thresholds for interpretation of heterogeneity were adopted as outlined in the Cochrane Handbook : 0% to 40% ‐ low, 30% to 60% ‐ moderate, 50% to 90% ‐ possible substantial, 75% to 100% ‐ considerable heterogeneity. Where the heterogeneity was deemed to be considerable we did not pool the results and provided a narrative assessment instead.
We conducted subgroup analyses by theoretical approach in all three groups (Pure Prevention cohorts, Change in Smoking Behaviour over time, and Point Prevalence of Smoking). We completed further subgroup analyses on Pure Prevention cohorts only (Group 1). This group was selected for additional subgroup analyses because these studies followed individual baseline never‐smokers through to follow‐up, and were expected to provide both the clearest indication of intervention effects and to have the lowest heterogeneity between studies. These analyses examined differences by gender, peer‐led versus adult‐led interventions, interventions focusing solely on tobacco versus interventions covering multiple areas, and the effects of adding booster sessions.
Sensitivity analysis
We conducted sensitivity analyses for all groups, to compare the overall study results against those studies with low or unclear risk of bias from attrition. We also viewed only those studies at low risk of bias from sequence generation, to assess whether the quality of randomisation had any impact on the overall results. We did not conduct sensitivity analyses for selective reporting, since all studies were assessed to be at low risk of bias, except for five studies which were rated as being at unclear risk and were not included in any analysis because of lack of data.
Results
Description of studies
Full details of all the trials are given in the Characteristics of included studies, Characteristics of excluded studies, and Characteristics of ongoing studies tables. Each study is identified by the name of the first author and year of publication of the main results paper. Additional references are listed together with this main publication under the study ID.
Included studies
The Characteristics of included studies table provides detail on each of the included studies. Overall, 133 cluster‐randomised controlled trials (C‐RCTs) and one RCT, giving a total of 200 arms and involving 428,293 participants from 25 different countries were included and placed in three groups (Note: ‘arms’ refers to different intervention groups within the RCTs, see Figure 1 and Appendix 6):
Flow chart of retrieval and identification of Group 1, 2 and 3 studies.
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Pure Prevention cohorts (Group 1): This group included 56 trials with 184,467 participants. Of these, 49 trials (73 arms) with 142,447 participants from 19 different countries provided analysable data. Twenty‐six were from the USA, four each from the Netherlands and the UK, three each from Canada, Germany and Italy, two each from China and Spain, and one each from Austria, Australia, Belgium, Czech Republic, Denmark, Finland, Greece, Portugal, South Africa, Sweden and Thailand. (N.B. Faggiano 2008 provided the comprehensive write‐up of results for a study set in Austria, Belgium, Germany, Greece, Italy, Spain and Sweden). See Appendix 7 for a list of Group 1 studies by country.
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Change in Smoking Behaviour over time (Group 2): Studies which provided change data. This included 16 trials with 57,577 participants, of which 15 trials (27 arms) with 45,555 participants provided analysable data. These studies came from three countries: 12 from the USA, two from India, one from Canada. See Appendix 8 for a list of studies by country.
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Point Prevalence of Smoking (Group 3): Studies which provided point prevalence data. This included one RCT and 65 C‐RCTs with 208,518 participants, of which one RCT and 24 C‐RCTs (39 arms) with 110,016 participants from 11 different countries provided usable data. Twelve were from the USA, two each from Australia, the Netherlands and the UK, and one each from France, Germany, India, Mexico, Norway, Romania and Sweden. See Appendix 9 for a list of studies by country. The only three studies (four arms) with intention‐to‐treat analysis are also in this group (McCambridge 2011; Sloboda 2009; Spoth 2002 (LST); Spoth 2002 (LST + SFP)).
Four studies (six arms) provided data to more than one group: Spoth 2001 (ISFP); Spoth 2001 (PDFY) to Pure Prevention cohorts (Group 1) and Change in Smoking Behaviour over time (Group 2), Ringwalt 2009a and Spoth 2002 (LST); Spoth 2002 (LST + SFP) to Pure Prevention cohorts (Group 1) and Point Prevalence of Smoking (Group 3), and Perry 2009 to Change in Smoking Behaviour over time (Group 2) and Point Prevalence of Smoking (Group 3). This is reflected in the total participant numbers and total trial numbers stated being reduced to take account of their multiple contributions.
Forty‐nine studies with 152,544 participants were not analysable for a variety of reasons:
(1) the publications did not provide data or only incomplete data on smoking status in the intervention and control groups at either baseline or follow‐ups;
(2) Numbers for intervention and control groups were not provided;
(3) the data were in an unusable format;
(4) the data were judged to be unreliable on closer scrutiny;
(5) the authors were not contactable to provide additional data;
(6) the authors were not able to provide these missing data.
Some studies focused on tobacco alone, and others on tobacco, alcohol, drugs, violence, cardiac health or policy change. The range of interventions was also heterogeneous. They included information about:
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Short‐ and long‐term consequences of smoking;
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Prevalence of smoking;
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Generic social skills;
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Tobacco‐, alcohol‐ and drug‐refusal skills;
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Interventions about tobacco included with interventions about risk‐taking, violence and carrying weapons;
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School interventions associated also with family and community interventions;
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Interventions to change school and state policies about tobacco availability;
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Classroom management and reading strategies for teachers;
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Culturally sensitive programmes, for example programmes for native North Americans.
The educational techniques were varied, and included lectures, quizzes, skits, collages, puppet plays, debates, role‐plays, making videos, discussions of videotaped role‐plays, films, interactive internet programmes, and meetings with athletes. Some studies compared interventions without a control group, and some included a control group in their comparisons. Some compared different types of presenters (teachers versus peers), and some compared videotaped to lecture presentations.
The presenters were usually the classroom teachers, but also included researchers, health educators, science teachers, undergraduate and graduate students, community members, uniformed police, and same‐age and older peers. The trials identified in this review are also heterogeneous in terms of duration of intervention (one hour to 36 classes spread over three years), and time from completion of intervention to final follow‐up (six months to 12 years).
The outcome measures most frequently chosen by authors were never‐smoking, and lifetime, monthly, weekly or daily smoking. Some studies used Pechacek's (Pechacek 1984) or Botvin's (Botvin 1980; Botvin 1984) composite indices, or constructed their own. Some studies classified students as current nonsmokers (which included never‐smokers, quitters and sometimes experimenters), and this heterogeneous category was the most difficult to assess. The authors were therefore contacted for clarification and/or new data sets. Few studies biochemically confirmed self reports at all stages of the research.
Excluded studies
Two hundred and two studies are excluded from the review. The majority (114) are not randomised controlled trials. Other reasons are that the intervention(s) was not in schools (N = 14), follow‐up was less than six months (N = 27), there were no smoking outcomes (N = 34), there were no baseline data (N = 2), the study was outside the age limits (N = 6), the study goal was smoking cessation only and did not include prevention, or there was no intervention (N = 5). These studies are listed in the Characteristics of excluded studies table, because the title and/or abstract had appeared to be relevance to this review.
Ongoing studies
Six studies are classified as ongoing. In four, some details and data are known from the studies, but are insufficient at this time to confirm inclusion in the review. The remaining two are expected to be included in a future update of the review, but the full results are currently awaiting publication. All six are listed in the Characteristics of ongoing studies table.
Risk of bias in included studies
(See Figure 2)
Risk of bias graph: review authors' judgements about each risk of bias item presented as percentages across all included studies, whether or not they had analysable data. The non‐coloured section of each bar represents different arms of multiple‐arm studies, for which risk of bias is assessed as a single measure for each study.
Selection bias: For the randomised control trials with analysable data, selection bias was assessed at low risk of bias in approximately half of the studies, and at unclear risk in almost all the remaining studies. Within the group of studies without analysable data, 12% were at low risk and 84% at unclear risk. The key area of uncertainty came from authors who mentioned only the word 'randomly', which resulted in a judgement of 'unclear.'
For the trials with analysable data, allocation concealment was assessed as being at unclear risk of bias in almost 95% of the studies. Those with no analysable data were either at unclear or at high risk of bias. This was predominantly because there was no comment in the study about allocation concealment. The Cochrane Handbook notes that:
"Cluster‐randomized trials often randomise all clusters at once, so lack of concealment of an allocation sequence should not usually be an issue. However, because small numbers of clusters are randomised, there is a possibility of chance baseline imbalance between the randomised groups, in terms of either the clusters or the individuals. Although not a form of bias as such, the risk of baseline differences can be reduced by using stratified or pair‐matched randomisation of clusters. Reporting of the baseline comparability of clusters, or statistical adjustment for baseline characteristics, can help reduce concern about the effects of baseline imbalance."
For each C‐RCT we verified, where possible, (1) if all clusters were randomised at the same time, (2) if samples were stratified on variables likely to influence tobacco‐use outcomes, (3) if clusters were pair‐matched, and (4) if there was baseline comparability between the intervention and control groups. Of the C‐RCTs with analysable data, 63% used pair matching and/or stratification.
Blinding: This was assessed as at unclear risk or unstated in almost all studies. Wood 2008, for 146 meta‐analyses involving 1346 trials, found that in trials with subjective outcomes, estimates of effect were exaggerated when there was unclear or inadequate concealment (ratio of odds ratios (ORs) 0.69, 95% confidence interval (CI) 0.59 to 0.82) and lack of blinding (ratio of ORs 0.75, 95% CI 0.61 to 0.93) but not in trials with objective outcomes. The outcomes in the studies in this review are objective smoking outcomes presented subjectively by adolescents. As Adams 2008 has shown, when adolescents' reports are objectively verified biochemically or they are asked to write their name on the questionnaire, their reports of weekly or monthly smoking rates significantly increase.
In this review, in most studies students were promised anonymity as they completed their questionnaires, but would most likely have known which study arm they were in, so that blinding was not feasible. In most studies the interventions were presented by classroom teachers, so that blinding of presenters was not possible. We cannot predict whether these factors would have increased or decreased the reporting of smoking rates.
Attrition bias: Across all study groups and also for those studies without analysable data, the percentage of studies assessed as being at low risk of attrition bias ranged from 40% to 50%, those at unclear risk from 40% to 58%, and those at high risk from 13% to 21%. There is no really satisfactory solution for missing data (Altman 2007). Patients excluded after randomisation are unlikely to be representative of those remaining (Nűesch 2009). The Cochrane Handbook advises mapping any methods for handling missing data closely to the known characteristics of the datasets, and to other datasets in the literature that are likely to have comparable outcomes. Adolescents who smoke may quit and re‐try, but are most likely to increase their frequency over time. There is thus some parallel with studies which tend to have worsening outcomes over time, such as lung cancer. Intention‐to‐treat solutions, such as baseline observation carried forward (BOCF), last observation carried forward (LOCF), and complete case analysis (excluding participants with incomplete outcome data) are therefore inappropriate because they require that the mechanisms governing drop‐out are independent of future unobserved measurements (Molenberghs 2004; Kenward 2009). Such independence is unlikely in this review because those who drop out are known to be more likely to be smokers and to have personal, family, friendship, social and cultural factors that promote smoking. Therefore, we did not replace missing data with our own estimates.
Selective reporting: For the trials with analysable data, the risk of bias from selective reporting was low for all the RCTs in Groups 1, 2 and 3, and for 90% of the studies which provided no analysable data.
Effects of interventions
Studies were classified into three groups according to how authors presented their data: Group 1 (Pure Prevention cohorts), Group 2 (Change in Smoking Behaviour over time), and Group 3 (Point Prevalence of Smoking). We contacted authors in Groups 2 and 3 and invited them to recompute their data to provide datasets of baseline never‐smokers; if they were unable to comply or did not reply we computed such datasets where we could. These results were then further analysed by duration of follow‐up and intervention category.
GROUP 1: PURE PREVENTION COHORT (49 C‐RCTs, 73 arms)
Comparison of all intervention curricula versus control, with duration of follow‐up of one year or less (See Analysis 1.1):
When the outcomes for all the trials testing any of the five different intervention curricula were pooled there was no overall effect (odds ratio (OR) 0.94, 95% confidence interval (CI) 0.85 to 1.05; I² = 0%; Analysis 1.1). The I² statistic for subgroup differences across all interventions was 44.1%, but within each intervention category heterogeneity was minimal.
One small trial (Howard 1996) which tested an information curriculum found no effect.
The combined social competence and social influences curricula (six RCTs/seven arms) showed a statistically significant effect in preventing the onset of smoking (OR 0.49, 95% CI 0.28 to 0.87; P = 0.01; I² = 0%; Analysis 1.1.3). However, for the social influences curricula (16 RCTs/25 arms) (OR 1.00, 95% CI 0.88 to 1.13; I² = 0%; Analysis 1.1.2) and the multimodal curricula (three RCTs/five arms) (OR 0.89, 95% CI 0.73 to 1.08; I² = 50%; Analysis 1.1.4), the results were not significant, with the 95% confidence interval including the line of no effect (= 1).
There was no RCT testing a social competence curriculum versus control with a follow‐up duration of less than one year.
One study with two arms, Figa‐Talamanca 1989 (F); Figa‐Talamanca 1989 (N.F), was included in the overall effect, but the intervention used did not fit into one of the five main intervention categories.
Sensitivity analyses:
Sensitivity analyses restricted to studies at low risk of bias in Group 1 found no differences from the all‐trials versions, apart from the trials of social competence and social influences curricula, which no longer demonstrated a significant effect, i.e. the all‐trials OR was 0.49 (95% CI 0.28 to 0.87), compared with the low risk of bias trial OR of 0.55 (95% CI 0.28 to 1.09; Analysis 2.1.3).
Comparison of all curricula versus control, with longest follow‐up period: [See Analysis 1.2]
When the outcomes for all the trials testing any of the five different intervention curricula were pooled there was a significant effect favouring the intervention (OR 0.88, 95% CI 0.82 to 0.96; P = 0.002; I² = 0%), with a mean risk reduction of 12%. (See Figure 3): Heterogeneity was 0%, except for the multimodal curricula trials (I² = 50%).
One C‐RCT testing information curricula detected a nonsignificant effect (OR 0.12, 95% CI 0.00 to 14.87; P not applicable).
Forest plot of comparison: 1 All curricula versus control, outcome: 1.2 Group 1: Pure Prevention cohorts (adjusted) at longest follow‐up.
Social competence curricula (five C‐RCTs/seven arms) versus control showed a statistically significant result in favour of the intervention (OR 0.52, 95 % CI 0.30 to 0.88; P = 0.02; I² = 0%; Analysis 1.2.2), as also did the combined social competence and social influences versus control (eight C‐RCTs/10 arms), (OR 0.50, 95% CI 0.28 to 0.87; P = 0.01; I² = 0%; Analysis 1.2.4).
There were no statistically significant differences for social influences programmes or multimodal curricula.
Four trials (six arms) contributed to the overall results, but not to the individual curricula (Brown 2002; Figa‐Talamanca 1989 (F); Figa‐Talamanca 1989 (N.F); Johnson 2009; Kellam 1998 (GBG); Kellam 1998 (ML)).
Sensitivity analyses:
Sensitivity analyses restricted to trials at low risk of selection bias demonstrated no differences from the all‐trials findings. Ranking by risk of attrition bias made little difference to the findings, apart from a widening of the confidence interval to include the line of no effect, i.e. all‐trials OR 0.88 (95% CI 0.82 to 0.96) versus low risk of bias OR 0.89 (95% CI 0.78 to 1.02).
GROUP 2: CHANGE IN SMOKING BEHAVIOUR OVER TIME (15 C‐RCTs, 27 arms)
Comparison of all curricula versus control, with duration of follow‐up of one year or less: (See Analysis 1.3)
The eight studies (15 arms) demonstrated a small statistically significant effect favouring the control group (standardised mean difference (SMD) 0.04, 95 % CI 0.02 to 0.06; P = 0.00001; I² =27%). This is similar to Pure Prevention cohorts (Group 1) combined social competence and social influences curriculum (only one C‐RCT (one arm)) found a significant effect favouring the intervention (SMD ‐0.38, 95%CI ‐0.59 to ‐0.17; P = 0.0004), but unlike Group 1 social influences curricula found a small statistically significant effect favouring the controls (six C‐RCTs/10 arms) (SMD 0.04, 95% CI 0.03 to 0.06; P = 0.00001; I² = 0%). There were no significant effects for information and social competence curricula.
Sensitivity analyses:
A sensitivity analysis restricted to trials at low risk of attrition bias demonstrated a nonsignificant effect.
Comparison of all curricula versus control, with longest follow‐up period: (See Analysis 1.4)
Fifteen C‐RCTs (27 arms) demonstrated a nonsignificant effect (SMD 0.01, 95% CI ‐0.00 to 0.02; P = 0.18; I² = 57%). Two C‐RCTs (five arms) that tested social competence curricula favoured the intervention (SMD ‐ 0.04, 95% CI ‐0.06 to ‐0.01; P = 0.01; I² = 0%). Ten C‐RCTs (16 arms) testing social influences curricula (SMD 0.05, 95% CI 0.03 to 0.06; P = 0.00001; I² = 0%) favoured the controls. There was no effect for information, combined social competence and social influences or multimodal curricula.
Sensitivity analyses:
Sensitivity analyses restricted to trials at low risk of attrition or selection bias demonstrated no important differences from the all‐trials findings.
GROUP 3: POINT PREVALENCE OF SMOKING (25 C‐RCTs, 39 arms): (See Analysis 1.5, Analysis 1.6)
The heterogeneity in this group of studies (for all interventions and for both follow‐up durations) was extremely high (minimum I² = 99%) and beyond what would be expected by chance alone. We have, therefore, not pooled these trials, but display them for reference
In the 16 studies (21 arms) that provided data at one year or less, eight out of 21 comparisons significantly favoured the controls (Analysis 1.5). This trend continued through longest follow‐up, with 20 of 25 studies (39 arms) significantly favouring the controls (Analysis 1.6).
Sensitivity analyses restricted to trials at low risk of selection bias or at low and unclear risk of attrition bias had no impact on the results.
Subgroup analyses (Pure Prevention cohort, Group 1 only)
Differences by gender (Analysis 3.1, Analysis 3.3):
At one year for the limited number of studies which presented data by gender, there was both a significant effect (OR 0.69, 95% CI 0.49 to 0.96; P = 0.04; I² = 30%) for females (seven arms), and for males (six arms) (OR 0.66, 95% CI 0.44 to 0.98; P = 0.04; I² = 30%). The largest effect was found in one study (De Vries 2003 (Finland)) which tested a multimodal curriculum (OR 0.32, 95% CI 0.16 to 0.65; P = 0.002) in males.
At longest follow‐up there were no statistically significant differences for females (nine arms) or males (eight arms).
Peer‐ versus adult‐led interventions:
Adult‐led interventions (29 arms) were not shown to be more effective up to one year than controls in any of the programmes, except for combined social competence and social influences curricula (OR 0.46, 95% CI 0.26 to 0.84; P = 0.01; I² = 0%). There was no overall effect for the peer‐led interventions (8 arms) compared to controls, although this only included social influences curricula tested by a single study (Botvin 1982) which offered a combined social competence and social influences curriculum (Analysis 6.1, Analysis 6.3).
In contrast, at longest follow‐up there were significant overall effects for adult‐led interventions (56 arms) compared to the control groups (OR 0.88, 95% CI 0.81 to 0.96; P = 0.002; I² = 17%), and significant effects for two of the four curricula tested: social competence (7 arms) (OR 0.52, 95% CI 0.30 to 0.88; P = 0.02, I² = 0%) and combined social competence and social influences (7 arms) (OR 0.47, 95% CI 0.26 to 0.84; P = 0.01, I² = 0%), but not for social influences or multimodal curricula. For peer‐led programmes (11 arms) compared to controls (Analysis 6.2) there were no statistically significant differences overall, nor for the three curricula tested (social influences, combined social competence and social influences and multimodal).
Four studies (six arms) which compared peer‐led and adult‐led interventions to controls were not included, either because it was not clear who delivered the programme (Conner 2010 (I); Conner 2010 (SE); Seal 2006) or because it was delivered online (Buller 2008 (Australia); Buller 2008 (USA); Prokhorov 2008).
Interventions focused on tobacco versus interventions covering multiple areas:
When the effectiveness of multifocal curricula (i.e. a combined focus on tobacco, drugs and alcohol prevention) was compared to control there was no overall effect at one year or at longest follow‐up. Only one curriculum, social competence (seven arms), showed a significant effect at longest follow‐up (OR 0.52, 95% CI 0.30 to 0.88; P = 0.02; I² = 0%; Analysis 5.2.2).
Curricula focused only on tobacco use prevention (26 arms) compared to controls showed no effect (OR 0.93, 95% CI 0.83 to 1.04) at one year, although there was an effect at longest follow‐up (42 arms) (OR 0.88, 95% CI 0.80 to 0.97; P = 0.01; I² = 20%; Analysis 5.4). None of the the three curricula tested at one year or at longest follow‐up (social influences, combined social competence and social influences, and multi‐modal) found significant differences.
Effect of adding booster sessions:
At one year or less there were no significant differences for curricula (36 arms) which did not include booster sessions, compared to controls (OR 0.94, 95% CI 0.85 to 1.05; Analysis 4.1), or at longest follow‐up (66 arms) (OR 0.90, 95% CI 0.83 to 0.97; P = 0.10; I² = 0%; Analysis 4.2).
For curricula which included booster sessions, there were no significant differences from controls at one year or less (four arms) (OR 0.70, 95% CI 0.40 to 1.07), but at longest follow‐up (seven arms) there was a significant difference (OR 0.73, 95% CI 0.55 to 0.98; Analysis 4.4).
The combined social competence and social influences curricula (OR 0.50, 95% CI 0.26 to 0.96; P = 0.04; I² = 0%) had a positive effect at one year or less (two arms) and also at longest follow‐up (three arms) (OR 0.51, 95% CI 0.27 to 0.96; P = 0.04; I² = 0%).
Discussion
Summary of Main Results
Outcomes are presented for three distinct groups: Pure Prevention cohorts of baseline never‐smokers, studies where authors presented results as Change in Smoking Behaviour over time, and studies where authors presented data as Point Prevalence of Smoking. Only four studies contributed to more than one group.
In the Pure Prevention cohort (Group 1), one might expect the clearest indication of whether smoking interventions prevent smoking, as studies followed the same cohort of never‐smoking individuals from baseline to follow‐ups. This group of cluster‐randomised controlled trials (C‐RCTs) with follow‐up of a year or less demonstrated no overall significant effect, with only the combined social competence and social influences curricula delivering positive results. Pooling the results from all the trials at longest follow‐up favoured the intervention groups (OR 0.88, 95% CI 0.82 to 0.96). This represents a risk reduction of 12% and suggests that interventions were more effective over a longer time period. The only intervention categories within this group that showed a statistically significant result were social competence and combined social competence and social influence curricula. This indicates that the success of the combined social competence and social influence curricula at one year was maintained over a longer period. There were no social competence intervention studies with one year or less of follow‐up for comparison.
Though pooled data suggest a significant effect in favour of the controls on Change in Smoking Behaviour over time (Group 2), the results are not incompatible with those of the Pure Prevention cohort studies (Group 1). Whilst the overall effect marginally favours the controls, there are similarities at intervention programme level to the results from the Pure Prevention cohort studies. This would be expected, since these studies, while measuring a change rate, follow the same groups of participants over time. Higher heterogeneity in this group could be explained by the differences between the participants (never‐smokers, experimenters and quitters) and between outcome measures.
Sensitivity analyses for Pure Prevention cohorts (Group 1) and Change in Smoking Behaviour over time (Group 2) for selection and attrition bias revealed no differences between studies at low risk and those at unclear or high risk.
In the Group 3 studies which present point prevalence smoking data, it was not possible to pool data due to the high level of heterogeneity, though the trends may have favoured the controls. The most likely explanation for the heterogeneity is that the same individuals are not consistently being measured over time, and thus point prevalence data are inadequate to measure the effectiveness of this type of intervention.
Subgroup analyses were only completed for the Pure Prevention cohorts (Group 1) data, and showed that:
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Gender: For the few studies that reported results by gender, there were positive significant results for both females and males with one year or less of follow‐up. However, within both groups only one intervention category (multimodal) in one study for males found a positive significant result.
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Peer‐led versus adult‐led interventions: There were no significant differences for studies at one year or less for peer‐led compared to adult‐led curricula, except for adult‐led combined social competence and social influences curricula. At longest follow‐up there were significant differences favouring adult‐led curricula, and for adult‐led social competence curricula and adult‐led combined social competence and social influences curricula.
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Multifocal versus tobacco‐only interventions: At one year or less there were no differences between multifocal and tobacco‐only programmes. However, at longest follow‐up tobacco‐only curricula had a significant effect, and within multifocal interventions the social competence returned positive findings.
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Booster sessions versus no boosters sessions: Major effort has been expended in many studies to provide booster sessions, expecting that they would reinforce the effects of the original programmes. At one year or less the presence or absence of boosters made no difference. Combined social competence and social influences curricula appeared to benefit from booster sessions in the medium and long term. This suggests that curricular orientation may be more important than providing booster sessions.
Overall completeness and applicability of evidence
The number of studies which provided no analysable data is large (49 C‐RCTs with 152,544 students), with seven C‐RCTs (42,020 students) from the Pure Prevention cohorts (Group 1), one C‐RCT (12,022 students) from the Change in Smoking Behaviour over time group (Group 2), and 41 C‐RCTs (98,502 students) from the Point Prevalence of Smoking group (Group 3). Twelve per cent of these trials are at low risk and 84% at unclear risk of selection bias, compared with approximately half at low risk and almost all the remaining studies at unclear risk in the trials with usable data. However, the percentages at low and unclear risk were similar for allocation, blinding, attrition and reporting biases. Our inability to include this large number of C‐RCTs and participants therefore excludes data of lower quality with respect to selection bias. A funnel plot (not shown) did not suggest publication bias in Pure Prevention cohorts or Change in Smoking Behaviour analyses.
Population:
Of the trials which provided analysable data, 56% were from North America (51% from the USA), 35% from Europe, 5% from Asia, 3% from Australia, and 1% from Africa. There is thus minimal representation from four of the six continents. In the US studies there is wide representation of urban and rural, socioeconomic, and ethnic groups. Few studies reported data separately by gender.
Interventions:
We placed no restrictions on the type of intervention that was included, provided it was school‐based. This resulted in a huge variety of interventions, which were analysed in six broad categories. A small number of interventions could not be classified, and although they are included in the overall analysis it was inappropriate to assess them as a separate category.
Social influence curricula were tested more than any other curricula in studies. In the Pure Prevention cohorts group, 63% of intervention arms at one year or less and 67% at longest follow‐up tested social influences interventions. The proportions in the Change in Smoking Behaviour over time group were 67% and 59% respectively. Only in the Change in Smoking Behaviour were social influence curricula found to be significant, and these favoured the controls.
Ideally, the review would have examined the positive effect of social competence or combined social competence and social influences further, by considering studies that made direct comparisons of these intervention types. However, although there were a few studies that explored comparisons between interventions, none of them considered these intervention types.
Outcomes:
The trials deployed a wide variety of outcome measures: never‐smoking; lifetime, monthly, weekly or daily smoking; numbers of cigarettes smoked during each of these time intervals; and indices such as Pechacek's (Pechacek 1984) or Botvin's (Botvin 1980; Botvin 1984). Some studies used the term 'current nonsmokers,' but this can include never‐smokers, experimenters and quitters, which can introduce a lack of clarity into any attempt to follow cohorts. The measures used most frequently are never‐smoking; smoking in the past 30 days and current nonsmoking.
Quality of the Evidence
The main strength of this review is the large number of included studies (134) and the number of participants (428,293). Although a large number of trials (85) with 275,749 participants provided analysable data, a limitation of this review is that 49 trials (152,544 participants; 37% of the total) were eligible, but did not provide sufficient data in their publications or did not provide the data after study authors were contacted. However, the data we could not include are deemed to be at greater risk of selection bias than the usable information.
For the Pure Prevention cohorts (Group 1) trials, it is worth noting that 49 studies (73 arms) with 142,447 participants were included in the analysis, representing 88% of all potential Pure Prevention cohorts trials.
Key methodological problems:
Key problems in some studies are a failure to describe robust methods of randomisation or allocation concealment, high rates of attrition, varying outcome measures for tobacco use, the use of 'current nonsmoker' as an outcome, failure to follow groups of never‐smokers, triers, and quitters separately over time, and failure to report basic data such as the numbers and smoking status in the intervention and control groups at baseline and follow‐ups. Our decision not to pool data from the Point Prevalence of Smoking trials arose from our assessment of point prevalence as an inadequate measure for reporting effects in these types of studies.
Consistency between the Pure Prevention cohorts (Group 1) and Change in Smoking Behaviour over time studies (Group 2) was good, but it was not possible to compare them with the Point Prevalence of Smoking studies (Group 3). Whilst many studies reported inadequately on their randomisation process and on attrition, sensitivity analyses suggest that these potential risks of bias did not have any real effect on the main findings for each group or intervention type.
Potential biases in the review process
One strength of this review is that the search was conducted across multiple electronic data bases, and included 'grey' literature, the searching of reference lists of articles, and consultation with experts. There were no limitations of date or language, and translations were obtained for any article as required. It is unlikely that this extensive search would have missed key trials.
Two authors independently reviewed all titles and abstracts and independently entered all data on Cochrane Tobacco Review Group data extraction forms. Extensive correspondence (over 600 emails) was undertaken with all study authors if data on risks of bias, the planning and conduct of the trial, numbers, stratification and pairing of clusters, baseline equivalence of intervention and control arms, and tobacco outcome status were not provided in the publications. Many study authors computed new databases of baseline never‐smokers for the review, or the reviewers computed this data.
Bias could have been introduced due to the high variability of outcome measures, although this has been reduced by dividing the studies into three groups and analysing the data for each group separately. The low heterogeneity in the Pure Prevention cohorts (Group 1) studies supports this approach. Bias may also have been introduced by certain assumptions made by the study authors in data extraction, and subsequent statistical analysis. This is particularly pertinent in the Point Prevalence of Smoking studies (Group 3), where we considered it inappropriate to pool the data.
Agreements and disagreements with other studies or reviews
There is no other comprehensive review of interventions in schools for comparison.
Flow chart of retrieval and identification of Group 1, 2 and 3 studies.
Risk of bias graph: review authors' judgements about each risk of bias item presented as percentages across all included studies, whether or not they had analysable data. The non‐coloured section of each bar represents different arms of multiple‐arm studies, for which risk of bias is assessed as a single measure for each study.
Forest plot of comparison: 1 All curricula versus control, outcome: 1.2 Group 1: Pure Prevention cohorts (adjusted) at longest follow‐up.
Comparison 1 All curricula versus control, Outcome 1 Group 1: Pure Prevention cohort (adjusted) ‐ 1 year or less.
Comparison 1 All curricula versus control, Outcome 2 Group 1: Pure Prevention cohort (adjusted) ‐ longest follow‐up.
Comparison 1 All curricula versus control, Outcome 3 Group 2: Change in Smoking Behaviour over time ‐ 1 year or less.
Comparison 1 All curricula versus control, Outcome 4 Group 2: Change in Smoking Behaviour over time ‐ longest follow‐up.
Comparison 1 All curricula versus control, Outcome 5 Group 3: Point Prevalence of Smoking ‐ 1 year or less.
Comparison 1 All curricula versus control, Outcome 6 Group 3: Point Prevalence of Smoking ‐ longest follow‐up.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 1 Low attrition ‐ 1 year or less.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 2 Low attrition ‐ longest follow‐up.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 3 Low & unclear attrition ‐ 1 year or less.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 4 Low & unclear attrition‐ longest follow‐up.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 5 Low selection bias ‐ 1 year or less.
Comparison 2 Group 1: Sensitivity analyses (adjusted), Outcome 6 Low selection bias ‐ longest follow‐up.
Comparison 3 Group 1: Gender analysis, Outcome 1 Female ‐ 1 year or less.
Comparison 3 Group 1: Gender analysis, Outcome 2 Female ‐ longest follow‐up.
Comparison 3 Group 1: Gender analysis, Outcome 3 Male ‐ 1 year or less.
Comparison 3 Group 1: Gender analysis, Outcome 4 Male ‐ longest follow‐up.
Comparison 4 Group 1: Booster sessions analysis, Outcome 1 No Booster sessions ‐ 1 year or less.
Comparison 4 Group 1: Booster sessions analysis, Outcome 2 No Booster sessions ‐ longest follow‐up.
Comparison 4 Group 1: Booster sessions analysis, Outcome 3 Boosters sessions ‐ 1 year or less.
Comparison 4 Group 1: Booster sessions analysis, Outcome 4 Booster sessions ‐ longest follow‐up.
Comparison 5 Group 1: Tobacco focus, Outcome 1 Multi foci ‐ 1 year or less.
Comparison 5 Group 1: Tobacco focus, Outcome 2 Multi foci ‐ longest follow‐up.
Comparison 5 Group 1: Tobacco focus, Outcome 3 Tobacco focused ‐ 1 year or less.
Comparison 5 Group 1: Tobacco focus, Outcome 4 Tobacco focused ‐ longest follow‐up.
Comparison 6 Group 1: Peer‐led analysis, Outcome 1 Peer‐led ‐ 1 year or less.
Comparison 6 Group 1: Peer‐led analysis, Outcome 2 Peer‐led ‐ longest follow‐up.
Comparison 6 Group 1: Peer‐led analysis, Outcome 3 Adult‐led ‐ 1 year or less.
Comparison 6 Group 1: Peer‐led analysis, Outcome 4 Adult‐led ‐ longest follow‐up.
Comparison 7 Group 2: Sensitivity analyses, Outcome 1 Low attrition ‐ 1 year or less.
Comparison 7 Group 2: Sensitivity analyses, Outcome 2 Low attrition ‐ > 1 year, longest follow‐up.
Comparison 7 Group 2: Sensitivity analyses, Outcome 3 Low & unclear attrition ‐ 1 year or less.
Comparison 7 Group 2: Sensitivity analyses, Outcome 4 Low & unclear attrition ‐ > 1 year, longest follow‐up.
Comparison 7 Group 2: Sensitivity analyses, Outcome 5 Low selection bias ‐ 1 year or less.
Comparison 7 Group 2: Sensitivity analyses, Outcome 6 Low selection bias ‐ > 1 year, longest follow‐up.
Comparison 8 Group 3: Sensitivity analyses, Outcome 1 Low attrition ‐ 1 year or less.
Comparison 8 Group 3: Sensitivity analyses, Outcome 2 Low attrition ‐ > 1 year, longest follow‐up.
Comparison 8 Group 3: Sensitivity analyses, Outcome 3 Low & unclear attrition ‐ 1 year or less.
Comparison 8 Group 3: Sensitivity analyses, Outcome 4 Low & unclear attrition ‐ > 1 year, longest follow‐up.
Comparison 8 Group 3: Sensitivity analyses, Outcome 5 Low selection bias ‐ 1 year or less.
Comparison 8 Group 3: Sensitivity analyses, Outcome 6 Low selection bias ‐ > 1 year, longest follow‐up.
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Group 1: Pure Prevention cohort (adjusted) ‐ 1 year or less Show forest plot | 40 | 32234 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.85, 1.05] |
| 1.1 Information giving curricula versus control | 1 | 100 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] |
| 1.2 Social influences curricula versus control | 25 | 20467 | Odds Ratio (Fixed, 95% CI) | 1.00 [0.88, 1.13] |
| 1.3 Combined social competence and social influences curricula versus control | 7 | 5370 | Odds Ratio (Fixed, 95% CI) | 0.49 [0.28, 0.87] |
| 1.4 Multimodal programmes versus control | 5 | 6000 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.73, 1.08] |
| 1.5 Other interventions | 2 | 297 | Odds Ratio (Fixed, 95% CI) | 2.49 [0.10, 61.80] |
| 2 Group 1: Pure Prevention cohort (adjusted) ‐ longest follow‐up Show forest plot | 73 | Odds Ratio (Random, 95% CI) | 0.88 [0.82, 0.96] | |
| 2.1 Information giving curricula versus control | 1 | Odds Ratio (Random, 95% CI) | 0.12 [0.00, 14.87] | |
| 2.2 Social competence curricula versus control | 7 | Odds Ratio (Random, 95% CI) | 0.52 [0.30, 0.88] | |
| 2.3 Social influences curricula versus control | 42 | Odds Ratio (Random, 95% CI) | 0.92 [0.84, 1.01] | |
| 2.4 Combined social competence and social influences versus control | 10 | Odds Ratio (Random, 95% CI) | 0.50 [0.28, 0.87] | |
| 2.5 Multimodal programmes versus control | 7 | Odds Ratio (Random, 95% CI) | 0.95 [0.64, 1.43] | |
| 2.6 Other interventions | 6 | Odds Ratio (Random, 95% CI) | 0.91 [0.50, 1.66] | |
| 3 Group 2: Change in Smoking Behaviour over time ‐ 1 year or less Show forest plot | 15 | 13137 | Std. Mean Difference (Fixed, 95% CI) | 0.04 [0.02, 0.06] |
| 3.1 Information giving curricula versus control | 1 | 1072 | Std. Mean Difference (Fixed, 95% CI) | 0.17 [‐0.04, 0.37] |
| 3.2 Social competence curricula versus control | 3 | 279 | Std. Mean Difference (Fixed, 95% CI) | 0.02 [‐1.19, 1.24] |
| 3.3 Social influences curricula versus control | 10 | 10689 | Std. Mean Difference (Fixed, 95% CI) | 0.04 [0.03, 0.06] |
| 3.4 Combined social competence and social influences curricula versus control | 1 | 1097 | Std. Mean Difference (Fixed, 95% CI) | ‐0.38 [‐0.59, ‐0.17] |
| 4 Group 2: Change in Smoking Behaviour over time ‐ longest follow‐up Show forest plot | 27 | Std. Mean Difference (Fixed, 95% CI) | 0.01 [‐0.00, 0.02] | |
| 4.1 Information giving curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.17 [‐0.04, 0.37] | |
| 4.2 Social competence versus control | 5 | Std. Mean Difference (Fixed, 95% CI) | ‐0.04 [‐0.06, ‐0.01] | |
| 4.3 Social influences curricula versus control | 16 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 4.4 Combined social competence and social influences curricula versus control | 3 | Std. Mean Difference (Fixed, 95% CI) | ‐0.02 [‐0.04, 0.00] | |
| 4.5 Multimodal programmes versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.11 [‐0.01, 0.22] | |
| 5 Group 3: Point Prevalence of Smoking ‐ 1 year or less Show forest plot | 21 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 5.1 Information giving curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5.2 Social influences curricula versus control | 15 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5.3 Combined social competence and social influence curricula versus control | 3 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5.4 Other interventions | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6 Group 3: Point Prevalence of Smoking ‐ longest follow‐up Show forest plot | 39 | Odds Ratio (Fixed, 95% CI) | Totals not selected | |
| 6.1 Information giving curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.2 Social competence | 1 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.3 Social influences curricula versus control | 23 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.4 Combined social competence and social influence curricula versus control | 10 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.5 Multimodal curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.6 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Low attrition ‐ 1 year or less Show forest plot | 13 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.75, 1.17] | |
| 1.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 1.2 Social influences curricula versus control | 10 | Odds Ratio (Fixed, 95% CI) | 1.00 [0.79, 1.27] | |
| 1.3 Combined social competence and social influences curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.09] | |
| 1.4 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.73 [0.00, 119016.25] | |
| 2 Low attrition ‐ longest follow‐up Show forest plot | 30 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.78, 1.02] | |
| 2.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 2.2 Social competence curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.57 [0.32, 1.02] | |
| 2.3 Social influences curricula versus control | 19 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.81, 1.08] | |
| 2.4 Combined social competence and social influences | 3 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.09] | |
| 2.5 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 1.03 [0.00, 433.58] | |
| 2.6 Other interventions | 1 | Odds Ratio (Fixed, 95% CI) | 0.86 [0.44, 1.69] | |
| 3 Low & unclear attrition ‐ 1 year or less Show forest plot | 31 | Odds Ratio (Fixed, 95% CI) | 0.92 [0.79, 1.07] | |
| 3.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 3.2 Social influences curricula versus control | 20 | Odds Ratio (Fixed, 95% CI) | 0.96 [0.82, 1.13] | |
| 3.3 Combined social competence and social influences versus control | 6 | Odds Ratio (Fixed, 95% CI) | 0.50 [0.28, 0.89] | |
| 3.4 Multimodal curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.72 [0.03, 19.98] | |
| 3.5 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 2.49 [0.10, 61.80] | |
| 4 Low & unclear attrition‐ longest follow‐up Show forest plot | 58 | Odds Ratio (Fixed, 95% CI) | 0.90 [0.82, 1.00] | |
| 4.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 4.2 Social competence curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.57 [0.32, 1.02] | |
| 4.3 Social influences curricula versus control | 35 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.84, 1.04] | |
| 4.4 Combined social competence and social influences | 8 | Odds Ratio (Fixed, 95% CI) | 0.50 [0.28, 0.89] | |
| 4.5 Multimodal curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 1.14 [0.21, 6.37] | |
| 4.6 Other interventions | 5 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.47, 1.69] | |
| 5 Low selection bias ‐ 1 year or less Show forest plot | 16 | Odds Ratio (Fixed, 95% CI) | 0.97 [0.80, 1.17] | |
| 5.1 Social influences curricula versus control | 12 | Odds Ratio (Fixed, 95% CI) | 1.02 [0.84, 1.24] | |
| 5.2 Combined social competence and social influences versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.10] | |
| 5.3 Multimodal curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.72 [0.03, 19.98] | |
| 6 Low selection bias ‐ longest follow‐up Show forest plot | 37 | Odds Ratio (Fixed, 95% CI) | 0.90 [0.80, 1.00] | |
| 6.1 Social competence curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.57 [0.32, 1.03] | |
| 6.2 Social influences curricula versus control | 24 | Odds Ratio (Fixed, 95% CI) | 0.92 [0.82, 1.03] | |
| 6.3 Combined social competence and social influences versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.10] | |
| 6.4 Multimodal curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 1.14 [0.21, 6.37] | |
| 6.5 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 0.88 [0.47, 1.66] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Female ‐ 1 year or less Show forest plot | 7 | Odds Ratio (Fixed, 95% CI) | 0.69 [0.49, 0.96] | |
| 1.1 Social influences curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.69 [0.41, 1.14] | |
| 1.2 Combined social competence and social influences curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.09] | |
| 1.3 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.82 [0.45, 1.47] | |
| 2 Female ‐ longest follow‐up Show forest plot | 9 | Odds Ratio (Fixed, 95% CI) | 0.82 [0.67, 1.00] | |
| 2.1 Social influences curricula versus control | 6 | Odds Ratio (Fixed, 95% CI) | 0.83 [0.66, 1.04] | |
| 2.2 Combined social competence and social influences versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.28, 1.07] | |
| 2.3 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.99 [0.57, 1.73] | |
| 3 Male ‐ 1 year or less Show forest plot | 6 | Odds Ratio (Fixed, 95% CI) | 0.66 [0.44, 0.98] | |
| 3.1 Social influences curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.92 [0.56, 1.52] | |
| 3.2 Combined social competence and social influences curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 1.15 [0.04, 37.54] | |
| 3.3 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.32 [0.16, 0.65] | |
| 4 Male ‐ longest follow‐up Show forest plot | 8 | Odds Ratio (Fixed, 95% CI) | 0.96 [0.77, 1.20] | |
| 4.1 Social influences curricula verus control | 6 | Odds Ratio (Fixed, 95% CI) | 0.97 [0.76, 1.23] | |
| 4.2 Combined social competence and social influences curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.77 [0.05, 11.85] | |
| 4.3 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.54, 1.58] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 No Booster sessions ‐ 1 year or less Show forest plot | 36 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.85, 1.05] | |
| 1.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 1.2 Social influences curricula versus control | 23 | Odds Ratio (Fixed, 95% CI) | 0.98 [0.86, 1.11] | |
| 1.3 Combined social competence and social influences curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.47 [0.14, 1.51] | |
| 1.4 Multimodal curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.73, 1.08] | |
| 1.5 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 2.49 [0.10, 61.80] | |
| 2 No Booster sessions ‐ longest follow‐up Show forest plot | 66 | Odds Ratio (Fixed, 95% CI) | 0.90 [0.83, 0.97] | |
| 2.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 2.2 Social competence curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.52 [0.30, 0.88] | |
| 2.3 Social influences curricula versus control | 38 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.85, 1.02] | |
| 2.4 Combined social competence and social influences curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.47 [0.15, 1.43] | |
| 2.5 Multimodal curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.83 [0.69, 1.01] | |
| 2.6 Other interventions | 6 | Odds Ratio (Fixed, 95% CI) | 0.91 [0.50, 1.66] | |
| 3 Boosters sessions ‐ 1 year or less Show forest plot | 4 | Odds Ratio (Fixed, 95% CI) | 0.70 [0.46, 1.07] | |
| 3.1 Social influences curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.90 [0.51, 1.56] | |
| 3.2 Combined social competence and social influences curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 0.50 [0.26, 0.96] | |
| 4 Booster sessions ‐ longest follow‐up Show forest plot | 7 | Odds Ratio (Fixed, 95% CI) | 0.73 [0.55, 0.98] | |
| 4.1 Social influences curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.81 [0.58, 1.12] | |
| 4.2 Combined social competence and social influences curricula versus control | 3 | Odds Ratio (Fixed, 95% CI) | 0.51 [0.27, 0.96] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Multi foci ‐ 1 year or less Show forest plot | 14 | Odds Ratio (Fixed, 95% CI) | 0.92 [0.74, 1.16] | |
| 1.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 1.2 Social influences curricula versus control | 9 | Odds Ratio (Fixed, 95% CI) | 0.99 [0.78, 1.27] | |
| 1.3 Combined social competence and social influences curricula versus control | 3 | Odds Ratio (Fixed, 95% CI) | 0.54 [0.28, 1.06] | |
| 1.4 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.72 [0.02, 22.85] | |
| 2 Multi foci ‐ longest follow‐up Show forest plot | 29 | Odds Ratio (Fixed, 95% CI) | 0.88 [0.77, 1.01] | |
| 2.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 2.2 Social competence curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.52 [0.30, 0.88] | |
| 2.3 Social influences curricula versus control | 14 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.82, 1.09] | |
| 2.4 Combined social competence and social influences curricula versus control | 6 | Odds Ratio (Fixed, 95% CI) | 0.55 [0.29, 1.04] | |
| 2.5 Multimodal curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.82 [0.03, 25.09] | |
| 3 Tobacco focused ‐ 1 year or less Show forest plot | 26 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.83, 1.04] | |
| 3.1 Social influence curricula versus control | 16 | Odds Ratio (Fixed, 95% CI) | 0.97 [0.84, 1.12] | |
| 3.2 Combined social competence and social influences curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.37 [0.12, 1.13] | |
| 3.3 Multimodal curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.73, 1.08] | |
| 3.4 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 2.49 [0.10, 61.80] | |
| 4 Tobacco focused ‐ longest follow‐up Show forest plot | 42 | Odds Ratio (Fixed, 95% CI) | 0.88 [0.80, 0.97] | |
| 4.1 Social influences curricula versus control | 28 | Odds Ratio (Fixed, 95% CI) | 0.91 [0.81, 1.02] | |
| 4.2 Combined social competence and social influences curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.37 [0.12, 1.13] | |
| 4.3 Multimodal curricula versus control | 4 | Odds Ratio (Fixed, 95% CI) | 0.83 [0.68, 1.00] | |
| 4.4 Other interventions | 6 | Odds Ratio (Fixed, 95% CI) | 0.91 [0.50, 1.66] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Peer‐led ‐ 1 year or less Show forest plot | 8 | Odds Ratio (Fixed, 95% CI) | 0.91 [0.56, 1.46] | |
| 1.1 Social influences curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.90 [0.56, 1.47] | |
| 1.2 Combined social competence and social influences curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.97 [0.11, 8.39] | |
| 2 Peer‐led ‐ longest follow‐up Show forest plot | 11 | Odds Ratio (Fixed, 95% CI) | 0.94 [0.61, 1.47] | |
| 2.1 Social influences curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.59, 1.47] | |
| 2.2 Combined social competence and social influences | 2 | Odds Ratio (Fixed, 95% CI) | 0.99 [0.12, 8.00] | |
| 2.3 Multimodal curricula versus control | 2 | Odds Ratio (Fixed, 95% CI) | 1.31 [0.16, 10.73] | |
| 3 Adult‐led ‐ 1 year or less Show forest plot | 29 | Odds Ratio (Fixed, 95% CI) | 0.93 [0.83, 1.03] | |
| 3.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 3.2 Social influences curricula versus control | 16 | Odds Ratio (Fixed, 95% CI) | 0.98 [0.86, 1.12] | |
| 3.3 Combined social competence and social influences curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.46 [0.26, 0.84] | |
| 3.4 Multimodal curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.89 [0.73, 1.08] | |
| 3.5 Other interventions | 2 | Odds Ratio (Fixed, 95% CI) | 2.49 [0.10, 61.80] | |
| 4 Adult‐led ‐ longest follow‐up Show forest plot | 56 | Odds Ratio (Fixed, 95% CI) | 0.88 [0.81, 0.96] | |
| 4.1 Information curricula versus control | 1 | Odds Ratio (Fixed, 95% CI) | 0.12 [0.00, 14.87] | |
| 4.2 Social competence curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.52 [0.30, 0.88] | |
| 4.3 Social influences curricula versus control | 30 | Odds Ratio (Fixed, 95% CI) | 0.92 [0.84, 1.01] | |
| 4.4 Combined social competence and social influences curricula versus control | 7 | Odds Ratio (Fixed, 95% CI) | 0.47 [0.26, 0.84] | |
| 4.5 Multimodal curricula versus control | 5 | Odds Ratio (Fixed, 95% CI) | 0.83 [0.68, 1.00] | |
| 4.6 Other interventions | 6 | Odds Ratio (Fixed, 95% CI) | 0.91 [0.50, 1.66] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Low attrition ‐ 1 year or less Show forest plot | 5 | Std. Mean Difference (Fixed, 95% CI) | ‐0.02 [‐3.01, 2.98] | |
| 1.1 Social influences curricula versus control | 5 | Std. Mean Difference (Fixed, 95% CI) | ‐0.02 [‐3.01, 2.98] | |
| 2 Low attrition ‐ > 1 year, longest follow‐up Show forest plot | 15 | Std. Mean Difference (Fixed, 95% CI) | ‐0.02 [‐0.05, 0.00] | |
| 2.1 Social competence curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | ‐0.04 [‐0.06, ‐0.01] | |
| 2.2 Social influences curricula versus control | 10 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [‐0.06, 0.16] | |
| 2.3 Combined social competence and social influences curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | ‐0.15 [‐0.36, 0.05] | |
| 2.4 Multimodal curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.11 [‐0.01, 0.22] | |
| 3 Low & unclear attrition ‐ 1 year or less Show forest plot | 13 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 3.1 Information curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.17 [‐0.04, 0.37] | |
| 3.2 Social competence curricula versus control | 3 | Std. Mean Difference (Fixed, 95% CI) | 0.02 [‐1.19, 1.24] | |
| 3.3 Social influences curricula versus control | 9 | Std. Mean Difference (Fixed, 95% CI) | 0.04 [0.03, 0.06] | |
| 3.4 Low & unclear attrition ‐ 1 year or less | 0 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4 Low & unclear attrition ‐ > 1 year, longest follow‐up Show forest plot | 25 | Std. Mean Difference (Fixed, 95% CI) | 0.01 [‐0.00, 0.02] | |
| 4.1 Information | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.17 [‐0.04, 0.37] | |
| 4.2 Social competence curricula versus control | 5 | Std. Mean Difference (Fixed, 95% CI) | ‐0.04 [‐0.06, ‐0.01] | |
| 4.3 Social influences curricula versus control | 15 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 4.4 Combined social competence and social influences curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | ‐0.02 [‐0.04, 0.00] | |
| 4.5 Multimodal curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.11 [‐0.01, 0.22] | |
| 5 Low selection bias ‐ 1 year or less Show forest plot | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 5.1 Social influences curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 6 Low selection bias ‐ > 1 year, longest follow‐up Show forest plot | 11 | Std. Mean Difference (Fixed, 95% CI) | 0.02 [0.01, 0.04] | |
| 6.1 Social competence curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | ‐0.04 [‐0.06, ‐0.01] | |
| 6.2 Social influences curricula versus control | 7 | Std. Mean Difference (Fixed, 95% CI) | 0.05 [0.03, 0.06] | |
| 6.3 Multimodal curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.11 [‐0.01, 0.22] | |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Low attrition ‐ 1 year or less Show forest plot | 14 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 1.1 Information curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 1.2 Social influences curricula versus control | 10 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 1.3 Combined social competence and social influences curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 1.4 Other interventions | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2 Low attrition ‐ > 1 year, longest follow‐up Show forest plot | 20 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 2.1 Information | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2.2 Social competence | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2.3 Social influences curricula versus control | 13 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2.4 Combined social competence and social influences curricula versus control | 3 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2.5 Multimodal curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 2.6 Other interventions | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 3 Low & unclear attrition ‐ 1 year or less Show forest plot | 17 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 3.1 Information curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 3.2 Social influences curricula versus control | 12 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 3.3 Combined social competence and social influences curricula versus control | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 3.4 Other interventions | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4 Low & unclear attrition ‐ > 1 year, longest follow‐up Show forest plot | 29 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 4.1 Information | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4.2 Social competence | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4.3 Social influences curricula versus control | 17 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4.4 Combined social competence and social influences curricula versus control | 8 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4.5 Multimodal curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 4.6 Other interventions | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5 Low selection bias ‐ 1 year or less Show forest plot | 9 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 5.1 Social influences curricula versus control | 6 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5.2 Combined social competence and social influences curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 5.3 Other interventions | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6 Low selection bias ‐ > 1 year, longest follow‐up Show forest plot | 19 | Std. Mean Difference (Fixed, 95% CI) | Totals not selected | |
| 6.1 Social influences curricula versus control | 9 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.2 Combined social competence and social influences curricula versus control | 7 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.3 Multimodal curricula versus control | 1 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
| 6.4 Other interventions | 2 | Std. Mean Difference (Fixed, 95% CI) | 0.0 [0.0, 0.0] | |
