Skip to main content
Key Specialties
Cardiology Diabetology Endocrinology Gastroenterology Geriatrics and Gerontology Gynecology and Obstetrics Hematology Infectious Disease Internal Medicine Nephrology Neurology Oncology Pediatrics Respiratory Medicine Rheumatology Surgery
Congress Coverage
2024 ACC Congress 2023 ESMO Congress 2023 EASD Congress 2023 ERS Congress 2023 ESC Congress
Clinical Collections
Advances in Alzheimer’s

Keynote webinar | Spotlight on medication adherence

LIVE: Thursday 27th June 2024, 18:00-19:30 (CEST)
Join our expert panel to discover why you need to understand the drivers of non-adherence in your patients, and how you can optimize medication adherence in your clinics to drastically improve patient outcomes.
ADVANCED SEARCH
Log in
Top
BMC Medical Research Methodology
Published in: BMC Medical Research Methodology 1/2012

Open Access 01-12-2012 | Research article

Bias and imprecision in posture percentile variables estimated from short exposure samples

Authors: Svend Erik Mathiassen, Jens Wahlström, Mikael Forsman

Published in: BMC Medical Research Methodology | Issue 1/2012

Login to get access

Abstract

Background

Upper arm postures are believed to be an important risk determinant for musculoskeletal disorder development in the neck and shoulders. The 10th and 90th percentiles of the angular elevation distribution have been reported in many studies as measures of neutral and extreme postural exposures, and variation has been quantified by the 10th-90th percentile range. Further, the 50th percentile is commonly reported as a measure of "average" exposure. These four variables have been estimated using samples of observed or directly measured postures, typically using sampling durations between 5 and 120 min.

Methods

The present study examined the statistical properties of estimated full-shift values of the 10th, 50th and 90th percentile and the 10th-90th percentile range of right upper arm elevation obtained from samples of seven different durations, ranging from 5 to 240 min. The sampling strategies were realized by simulation, using a parent data set of 73 full-shift, continuous inclinometer recordings among hairdressers. For each shift, sampling duration and exposure variable, the mean, standard deviation and sample dispersion limits (2.5% and 97.5%) of all possible sample estimates obtained at one minute intervals were calculated and compared to the true full-shift exposure value.

Results

Estimates of the 10th percentile proved to be upward biased with limited sampling, and those of the 90th percentile and the percentile range, downward biased. The 50th percentile was also slightly upwards biased. For all variables, bias was more severe with shorter sampling durations, and it correlated significantly with the true full-shift value for the 10th and 90th percentiles and the percentile range. As expected, shorter samples led to decreased precision of the estimate; sample standard deviations correlated strongly with true full-shift exposure values.

Conclusions

The documented risk of pronounced bias and low precision of percentile estimates obtained from short posture samples presents a concern in ergonomics research and practice, and suggests that alternative, unbiased exposure variables should be considered if data collection resources are restricted.
Appendix
Available only for authorised users
Literature
1.
Li G, Buckle P: Current techniques for assessing physical exposure to work-related musculoskeletal risks, with emphasis on posture-based methods. Ergonomics. 1999, 42: 674-695. 10.1080/001401399185388.CrossRefPubMed
2.
van der Beek AJ, Frings-Dresen MHW: Assessment of mechanical exposure in ergonomic epidemiology. Occup Environ Med. 1998, 55: 291-299. 10.1136/oem.55.5.291.CrossRefPubMedPubMedCentral
3.
David CG: Ergonomic methods for assessing exposure to risk factors for work-related musculoskeletal disorders. Occup Med (Lond). 2005, 55: 190-199. 10.1093/occmed/kqi082.CrossRef
4.
Takala EP, Pehkonen I, Forsman M, Hansson G-Å, Mathiassen SE, Neumann WP, Sjøgaard G, Veiersted KB, Westgaard RH, Winkel J: Systematic evaluation of observational methods assessing biomechanical exposures at work. Scand J Work Environ Health. 2010, 36: 3-24. 10.5271/sjweh.2876.CrossRefPubMed
5.
Punnett L, Fine LJ, Monroe Keyserling W, Herrin GD, Chaffin DB, Punnett L, Fine LJ, Monroe Keyserling W, Herrin GD, Chaffin DB: Shoulder disorders and postural stress in automobile assembly work. Scand J Work Environ Health. 2000, 26: 283-291. 10.5271/sjweh.544.CrossRefPubMed
6.
Svendsen SW, Bonde JP, Mathiassen SE, Steengaard-Pedersen K, Frich LH: Work-related shoulder disorders: quantitative exposure-response relationships with reference to arm posture. Occup Environ Med. 2004, 61: 844-853. 10.1136/oem.2003.010637.CrossRefPubMedPubMedCentral
7.
Jensen C, Nilsen K, Hansen K, Westgaard R: Trapezius muscle load as a risk indicator for occupational shoulder-neck complaints. Int Arch Occup Environ Health. 1993, 64: 415-423. 10.1007/BF00517947.CrossRefPubMed
8.
Veiersted KB, Westgaard RH, Andersen P: Electromyographic evaluation of muscular work pattern as a predictor of trapezius myalgia. Scand J Work Environ Health. 1993, 19: 284-290. 10.5271/sjweh.1472.CrossRefPubMed
9.
Kazmierczak K, Mathiassen SE, Forsman M, Winkel J: An integrated analysis of ergonomics and time consumption in Swedish 'craft-type' car disassembly. Appl Ergon. 2005, 36: 263-273. 10.1016/j.apergo.2005.01.010.CrossRefPubMed
10.
Aarås A, Fostervold KI, Ro O, Thoresen M, Larsen S: Postural load during VDU work: a comparison between various work postures. Ergonomics. 1997, 40: 1255-1268. 10.1080/001401397187496.CrossRefPubMed
11.
Jensen C, Finsen L, Hansen K, Christensen H: Upper trapezius muscle activity patterns during repetitive manual material handling and work with a computer mouse. J Electromyogr Kinesiol. 1999, 9: 317-325. 10.1016/S1050-6411(99)00007-3.CrossRefPubMed
12.
Wahlström J, Mathiassen SE, Liv P, Hedlund P, Forsman M, Ahlgren C: Upper arm postures and movements in female hairdressers across four full working days. Ann Occup Hyg. 2010, 54: 584-594. 10.1093/annhyg/meq028.CrossRefPubMed
13.
Arvidsson I, Hansson G-Å, Mathiassen SE, Skerfving S: Changes in physical workload with implementation of mouse-based information technology in air traffic control. Int J Ind Ergon. 2006, 36: 613-622. 10.1016/j.ergon.2006.03.002.CrossRef
14.
Juul-Kristensen B, Hansson G-Å, Fallentin N, Andersen JH, Ekdahl C: Assessment of work postures and movements using a video-based observation method and direct technical measurements. Appl Ergon. 2001, 32: 517-524. 10.1016/S0003-6870(01)00017-5.CrossRefPubMed
15.
Delisle A, Larivière C, Plamondon A, Imbeau D: Comparison of three computer office workstations offering forearm support: impact on upper limb posture and muscle activation. Ergonomics. 2006, 49: 139-160. 10.1080/10610270500450739.CrossRefPubMed
16.
Vasseljen O, Westgaard RH: Arm and trunk posture during work in relation to shoulder and neck pain and trapezius activity. Clin Biomech. 1997, 12: 22-31. 10.1016/S0268-0033(97)00048-X.CrossRef
17.
Bao S, Mathiassen SE, Winkel J: Ergonomic effects of a management-based rationalization in assembly work - a case study. Appl Ergon. 1996, 27: 89-99. 10.1016/0003-6870(95)00063-1.CrossRefPubMed
18.
Balogh I, Ohlsson K, Hansson G-Å, Engström T, Skerfving S: Increasing the degree of automation in a production system: consequences for the physical workload. Int J Ind Ergon. 2006, 36: 353-365. 10.1016/j.ergon.2004.09.009.CrossRef
19.
Unge J, Ohlsson K, Nordander C, Hansson G-Å, Skerfving S, Balogh I: Differences in physical workload, psychosocial factors and musculoskeletal disorders between two groups of female hospital cleaners with two diverse organizational models. Int Arch Occup Environ Health. 2007, 81: 209-220. 10.1007/s00420-007-0208-x.CrossRefPubMed
20.
Forsman M, Hansson G-Å, Medbo L, Asterland P, Engström T: A method for evaluation of manual work using synchronised video recordings and physiological measurements. Appl Ergon. 2002, 33: 533-540. 10.1016/S0003-6870(02)00070-4.CrossRefPubMed
21.
Jonker D, Rolander B, Balogh I: Relation between perceived and measured workload obtained by long-term inclinometry among dentists. Appl Ergon. 2009, 40: 309-315. 10.1016/j.apergo.2008.12.002.CrossRefPubMed
22.
Åkesson I, Hansson G-Å, Balogh I, Moritz U, Skerfving S: Quantifying work load in neck, shoulders and wrists in female dentists. Int Arch Occup Environ Health. 1997, 69: 461-474. 10.1007/s004200050175.CrossRefPubMed
23.
Hansson G-Å, Arvidsson I, Ohlsson K, Nordander C, Mathiassen SE, Skerfving S, Balogh I: Precision of measurements of physical workload during standardised manual handling. Part II: Inclinometry of head, upper back, neck and upper arms. J Electromyogr Kinesiol. 2006, 16: 125-136. 10.1016/j.jelekin.2005.06.009.CrossRefPubMed
24.
Hansson G-Å, Balogh I, Ohlsson K, Granqvist L, Nordander C, Arvidsson I, Åkesson I, Unge J, Rittner R, Strömberg U, et al: Physical workload in various types of work: Part II. Neck, shoulder and upper arm. Int J Ind Ergon. 2010, 40: 267-281. 10.1016/j.ergon.2009.11.002.CrossRef
25.
Veiersted KB, Gould KS, Osterås N, Hansson G-Å: Effect of an intervention addressing working technique on the biomechanical load of the neck and shoulders among hairdressers. Appl Ergon. 2008, 39: 183-190. 10.1016/j.apergo.2007.05.007.CrossRefPubMed
26.
Christmansson M, Medbo L, Hansson G-Å, Ohlsson K, Unge Byström J, Möller T, Forsman M: A case study of a principally new way of materials kitting - an evaluation of time consumption and physical workload. Int J Ind Ergon. 2002, 30: 49-65. 10.1016/S0169-8141(02)00077-X.CrossRef
27.
Fethke NB, Gant LC, Gerr F: Comparison of biomechanical loading during use of conventional stud welding equipment and an alternate system. Appl Ergon. 2011, 42: 725-734. 10.1016/j.apergo.2010.11.007.CrossRefPubMed
28.
Jonsson B: Measurement and evalution of local muscular strain in the shoulder during constrained work. J Hum Ergol. 1982, 11: 73-88. 10.1016/S0047-2484(82)80033-X.
29.
Jonsson B: Kinesiology. With special reference to electromyographic kinesiology. Electroencephalogr Clin Neurophysiol Suppl. 1978, 34: 417-428.PubMed
30.
Mathiassen SE, Burdorf A, van der Beek AJ: Statistical power and measurement allocation in ergonomic intervention studies assessing upper trapezius EMG amplitude. A case study of assembly work. J Electromyogr Kinesiol. 2002, 12: 27-39. 10.1016/S1050-6411(01)00031-1.CrossRef
31.
Aarås A, Westgaard RH, Stranden E: Postural angles as an indicator of postural load and muscular injury in occupational work situations. Ergonomics. 1988, 31: 915-933. 10.1080/00140138808966731.CrossRefPubMed
32.
To WM, Ip RC, Lam GC, Yau CT: A multiple regression model for urban traffic noise in Hong Kong. J Acoust Soc Am. 2002, 112: 551-556. 10.1121/1.1494803.CrossRefPubMed
33.
National Research Council: Musculoskeletal Disorders and the Workplace. 2001, Washington DC: National Academies Press
34.
Sluiter JK, Rest KM, Frings-Dresen MHW: Criteria document for evaluating the work-relatedness of upper-extremity musculoskeletal disorders. Scand J Work Environ Health. 2001, 27 (Suppl 1): 1-102.CrossRefPubMed
35.
Mathiassen SE: Diversity and variation in biomechanical exposure: what is it, and why would we like to know?. Appl Ergon. 2006, 37: 419-427. 10.1016/j.apergo.2006.04.006.CrossRefPubMed
36.
Straker L, Coleman J, Skoss R, Maslen BA, Burgess-Limerick R, Pollock C: A comparison of posture and muscle activity during tablet computer, desktop computer and paper use by young children. Ergonomics. 2008, 51: 540-555. 10.1080/00140130701711000.CrossRefPubMed
37.
Ciccarelli M, Straker L, Mathiassen SE, Pollock C: ITkids Part II: variation of postures and muscle activity in children using different information and communication technologies. Work. 2011, 38: 413-427.PubMed
38.
Bernmark E, Forsman M, Wiktorin C: Head movements during two computer tasks assessed by accelerometry. Appl Ergon. 2011, 42: 309-313. 10.1016/j.apergo.2010.07.006.CrossRefPubMed
39.
Dahlberg R, Karlqvist L, Bildt C, Nykvist K: Do work technique and musculoskeletal symptoms differ between men and women performing the same type of work tasks?. Appl Ergon. 2004, 35: 521-529. 10.1016/j.apergo.2004.06.008.CrossRefPubMed
40.
Svendsen SW, Mathiassen SE, Bonde JP: Task-based exposure assessment in ergonomic epidemiology - a study of upper arm elevation in the jobs of machinists, car mechanics, and house painters. Occup Environ Med. 2005, 62: 18-26. 10.1136/oem.2004.015966.CrossRefPubMedPubMedCentral
41.
Leijon O, Bernmark E, Karlqvist L, Härenstam A: Awkward work postures: association with occupational gender segregation. Am J Ind Med. 2005, 47: 381-393. 10.1002/ajim.20166.CrossRefPubMed
42.
Burdorf A, Verburgh A, Elders L: Time-dependent variation in back load of workers in a dairy factory. Occup Hyg. 1994, 1: 199-206.
43.
van der Beek AJ, Kuiper JI, Dawson M, Burdorf A, Bongers PM, Frings-Dresen MHW: Sources of variance in exposure to nonneutral trunk postures in varying work situations. Scand J Work Environ Health. 1995, 21: 215-222. 10.5271/sjweh.30.CrossRefPubMed
44.
Mathiassen SE, Möller T, Forsman M: Variability in mechanical exposure within and between individuals performing a highly constrained industrial work task. Ergonomics. 2003, 46: 800-824. 10.1080/0014013031000090125.CrossRefPubMed
45.
Allread WG, Marras WS, Burr DL: Measuring trunk motions in industry: variability due to task factors, individual differences, and the amount of data collected. Ergonomics. 2000, 43: 691-701. 10.1080/001401300404670.CrossRefPubMed
46.
Hoozemans MJM, Burdorf A, van der Beek AJ, Frings-Dresen MHW, Mathiassen SE: Group-based measurement strategies in exposure assessment explored by bootstrapping. Scand J Work Environ Health. 2001, 27: 125-132. 10.5271/sjweh.599.CrossRefPubMed
47.
Paquet V, Punnett L, Woskie S, Buchholz B: Reliable exposure assessment strategies for physical ergonomics stressors in construction and other non-routinized work. Ergonomics. 2005, 48: 1200-1219. 10.1080/00140130500197302.CrossRefPubMed
48.
Paquet V, Mathiassen SE, Dempsey PG: Video-based ergonomic job analysis. A practitioner's guide. Prof Saf. 2006, 11: 27-35.
49.
Fethke NB, Anton D, Cavanaugh JE, Gerr F, Cook TM: Bootstrap exploration of the duration of surface electromyography sampling in relation to the precision of exposure estimation. Scand J Work Environ Health. 2007, 33: 358-367. 10.5271/sjweh.1155.CrossRefPubMed
50.
Liv P, Mathiassen SE, Svendsen SW: Theoretical and empirical efficiency of sampling strategies for estimating upper arm elevation. Ann Occup Hyg. 2011, 55: 436-449. 10.1093/annhyg/meq095.CrossRefPubMed
51.
Mathiassen SE, Paquet V: The ability of limited exposure sampling to detect effects of interventions that reduce the occurrence of pronounced trunk inclination. Appl Ergon. 2010, 41: 295-304. 10.1016/j.apergo.2009.08.006.CrossRefPubMed
52.
Mathiassen SE, Burdorf A, van der Beek AJ, Hansson G-Å: Efficient one-day sampling of mechanical job exposure data - a study based on upper trapezius activity in cleaners and office workers. Am Ind Hyg Assoc J. 2003, 64: 196-211. 10.1080/15428110308984809.CrossRef
53.
Kazmierczak K, Mathiassen SE, Neumann WP, Winkel J: Observer reliability of industrial activity analysis based on video recordings. Int J Ind Ergon. 2006, 36: 275-282. 10.1016/j.ergon.2005.12.006.CrossRef
54.
Jackson JA, Mathiassen SE, Dempsey PG: Methodological variance associated with normalization of occupational upper trapezius EMG using submaximal reference contractions. J Electromyogr Kinesiol. 2009, 19: 416-427. 10.1016/j.jelekin.2007.11.004.CrossRefPubMed
55.
Liv P, Mathiassen SE, Svendsen SW: Accuracy and precision of variance components in occupational posture recordings: a simulation study of different data collection strategies. BMC Med Res Methodol. submitted
56.
Trask C, Teschke K, Morrison J, Johnson PW, Village J, Koehoorn M: How long is long enough? Evaluating sampling durations for low back EMG assessment. J Occup Environ Hyg. 2008, 5: 664-670. 10.1080/15459620802313840.CrossRefPubMed
57.
Hansson G-Å, Asterland P, Holmer N-G, Skerfving S: Validity and reliability of triaxial accelerometers for inclinometry in posture analysis. Med Biol Eng Comput. 2001, 39: 405-413. 10.1007/BF02345361.CrossRefPubMed
58.
Bland JM, Altman DG: Measuring agreement in method comparison studies. Stat Methods Med Res. 1999, 8: 135-160. 10.1191/096228099673819272.CrossRefPubMed
59.
Statistics with Confidence: Confidence intervals and statistical guidelines. Edited by: Altman DG, Machin D, Bryant T, Gardner S. 2000, Hoboken: Wiley/BMJ Books, 2
60.
Unge Byström J, Hansson G-Å, Rylander L, Ohlsson K, Källrot G, Skerfving S: Physical workload on neck and upper limb using two CAD applications. Appl Ergon. 2002, 33: 63-74. 10.1016/S0003-6870(01)00044-8.CrossRefPubMed
61.
Möller T, Mathiassen SE, Franzon H, Kihlberg S: Job enlargement and mechanical exposure variability in cyclic assembly work. Ergonomics. 2004, 47: 19-40. 10.1080/0014013032000121651.CrossRefPubMed
62.
Gaudart C: Conditions for maintaining ageing operators at work - a case study conducted at an automobile manufacturing plant. Appl Ergon. 2000, 31: 453-462. 10.1016/S0003-6870(00)00024-7.CrossRefPubMed
63.
Gold JE, Park JS, Punnett L: Work routinization and implications for ergonomic exposure assessment. Ergonomics. 2006, 49: 12-27. 10.1080/00140130500356643.CrossRefPubMed
64.
Hye-Knudsen CT, Schibye B, Hjortskov N, Fallentin N: Trunk motion characteristics during different patient handling tasks. Int J Ind Ergon. 2004, 33: 327-337. 10.1016/j.ergon.2003.09.007.CrossRef
65.
Burdorf A: Reducing random measurement error in assessing postural load on the back in epidemiological surveys. Scand J Work Environ Health. 1995, 21: 15-23. 10.5271/sjweh.3.CrossRefPubMed
66.
Tielemans E, Kupper LL, Kromhout H, Heederik D, Houba R: Individual-based and group-based occupational exposure assessment: some equations to evaluate different strategies. Ann Occup Hyg. 1998, 42: 115-119.CrossRefPubMed
67.
Seixas NS, Sheppard L: Maximizing accuracy and precision using individual and grouped exposure assessments. Scand J Work Environ Health. 1996, 22: 94-101. 10.5271/sjweh.116.CrossRefPubMed
68.
Reeves GK, Cox DR, Darby SC, Whitley E: Some aspects of measurement error in explanatory variables for continuous and binary regression models. Stat Med. 1998, 17: 2157-2177. 10.1002/(SICI)1097-0258(19981015)17:19<2157::AID-SIM916>3.0.CO;2-F.CrossRefPubMed
69.
Spiegelman D, McDermott A, Rosner B: Regression calibration method for correcting measurement-error bias in nutritional epidemiology. Am J Clin Nutr. 1997, 65 (4 suppl): 1179S-1186S.PubMed
70.
Stram DO, Hankin JH, Wilkens LR, Pike MC, Monroe KR, Park S, Henderson BE, Nomura AMY, Earle ME, Nagamine FS, et al: Calibration of the dietary questionnaire for a multiethnic cohort in Hawaii and Los Angeles. Am J Epidemiol. 2000, 151: 358-370.CrossRefPubMedPubMedCentral
71.
Rosner B, Gore R: Measurement error correction in nutritional epidemiology based on individual foods, with application to the relation of diet to breast cancer. Am J Epidemiol. 2001, 154: 827-835. 10.1093/aje/154.9.827.CrossRefPubMed
72.
Symanski E, Rappaport SM: An investigation of the dependence of exposure variability on the interval between measurements. Ann Occup Hyg. 1994, 38: 361-372. 10.1093/annhyg/38.4.361.CrossRefPubMed
73.
Lampa EG, Nilsson L, Liljelind IE, Bergdahl IA: Optimizing occupational exposure measurement strategies when estimating the log-scale arithmetic mean value - an example from the reinforced plastics industry. Ann Occup Hyg. 2006, 50: 371-377. 10.1093/annhyg/mei078.CrossRefPubMed
74.
Fransson-Hall C, Gloria R, Kilbom Å, Winkel J, Karlqvist L, Wiktorin C, Stockholm Music I Study Group: A portable ergonomic observation method (PEO) for computerized on-line recording of postures and manual handling. Appl Ergon. 1995, 26: 93-100. 10.1016/0003-6870(95)00003-U.CrossRefPubMed
75.
Kilbom Å, Persson J: Work technique and its consequences for musculoskeletal disorders. Ergonomics. 1987, 30: 273-279. 10.1080/00140138708969706.CrossRefPubMed
76.
Punnett L, Fine L, Keyserling WM, Herrin GD, Chaffin D: Back disorders and non-neutral trunk postures of automobile assembly workers. Scand J Work Environ Health. 1991, 17: 337-346. 10.5271/sjweh.1700.CrossRefPubMed
77.
Häkkänen M, Viikari-Juntura E, Takala E-P: Effects of changes in work methods on musculoskeletal load. An intervention study in the trailer assembly. Appl Ergon. 1997, 28: 99-108. 10.1016/S0003-6870(96)00040-3.CrossRefPubMed
78.
Denis D, Lortie M, Rossignol M: Observation procedures characterizing occupational physical activities: critical review. Int J Occup Safety Ergon. 2000, 6: 463-491.CrossRef
79.
Bao S, Howard N, Spielholz P, Silverstein B, Polissar N: Interrater reliability of posture observations. Hum Factors. 2009, 51: 292-309. 10.1177/0018720809340273.CrossRefPubMed
80.
Hooftman WE, van der Beek AJ, van de Wal BG, Knol DL, Bongers PM, Burdorf A, van Mechelen W: Equal task, equal exposure? Are men and women with the same tasks equally exposed to awkward working postures?. Ergonomics. 2009, 52: 1079-1086. 10.1080/00140130902915921.CrossRefPubMed
81.
Keyserling M: Postural analysis of the trunk and shoulders in simulated real time. Ergonomics. 1986, 29: 569-583. 10.1080/00140138608968292.CrossRefPubMed
82.
Bao S, Howard N, Spielholz P, Silverstein B: Two posture analysis approaches and their application in a modified rapid upper limb assessment evaluation. Ergonomics. 2009, 50: 2118-2136.CrossRef
Metadata
Title
Bias and imprecision in posture percentile variables estimated from short exposure samples
Authors
Svend Erik Mathiassen
Jens Wahlström
Mikael Forsman
Publication date
01-12-2012
Publisher
BioMed Central
Published in
BMC Medical Research Methodology / Issue 1/2012
Electronic ISSN: 1471-2288
DOI
https://doi.org/10.1186/1471-2288-12-36

Other articles of this Issue 1/2012

BMC Medical Research Methodology 1/2012 Go to the issue

Debate

'You give us rangoli, we give you talk': using an art-based activity to elicit data from a seldom heard group

Research article

Adjustment for reporting bias in network meta-analysis of antidepressant trials

Technical advance

Translation equations to compare ActiGraph GT3X and Actical accelerometers activity counts

Research article

Factor structure and psychometric properties of the trier inventory for chronic stress (TICS) in a representative german sample

Research article

Telephone follow-up to a mail survey: when to offer an interview compared to a reminder call

Research article

Designing cost-efficient randomized trials by using flexible recruitment strategies