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Open Access 01-12-2019 | Methodology

Many continuous variables should be analyzed using the relative scale: a case study of β₂-agonists for preventing exercise-induced bronchoconstriction

Authors: Harri Hemilä, Jan O. Friedrich

Published in: Systematic Reviews | Issue 1/2019

Abstract

Background

The relative scale adjusts for baseline variability and therefore may lead to findings that can be generalized more widely. It is routinely used for the analysis of binary outcomes but only rarely for continuous outcomes. Our objective was to compare relative vs absolute scale pooled outcomes using data from a recently published Cochrane systematic review that reported only absolute effects of inhaled β₂-agonists on exercise-induced decline in forced-expiratory volumes in 1 s (FEV₁).

Methods

From the Cochrane review, we selected placebo-controlled cross-over studies that reported individual participant data (IPD). Reversal in FEV₁ decline after exercise was modeled as a mean uniform percentage point (pp) change (absolute effect) or average percent change (relative effect) using either intercept-only or slope-only, respectively, linear mixed-effect models. We also calculated the pooled relative effect estimates using standard random-effects, inverse-variance-weighting meta-analysis using study-level mean effects.

Results

Fourteen studies with 187 participants were identified for the IPD analysis. On the absolute scale, β₂-agonists decreased the exercise-induced FEV₁ decline by 28 pp., and on the relative scale, they decreased the FEV₁ decline by 90%. The fit of the statistical model was significantly better with the relative 90% estimate compared with the absolute 28 pp. estimate. Furthermore, the median residuals (5.8 vs. 10.8 pp) were substantially smaller in the relative effect model than in the absolute effect model. Using standard study-level meta-analysis of the same 14 studies, β₂-agonists reduced exercise-induced FEV₁ decline on the relative scale by a similar amount: 83% or 90%, depending on the method of calculating the relative effect.

Conclusions

Compared with the absolute scale, the relative scale captures more effectively the variation in the effects of β₂-agonists on exercise-induced FEV₁-declines. The absolute scale has been used in the analysis of FEV₁ changes and may have led to sub-optimal statistical analysis in some cases. The choice between the absolute and relative scale should be determined based on biological reasoning and empirical testing to identify the scale that leads to lower heterogeneity.

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Title: Many continuous variables should be analyzed using the relative scale: a case study of β2-agonists for preventing exercise-induced bronchoconstriction
Authors: Harri Hemilä
Jan O. Friedrich
Publication date: 01-12-2019
Publisher: BioMed Central
Published in: Systematic Reviews / Issue 1/2019
Electronic ISSN: 2046-4053
DOI: https://doi.org/10.1186/s13643-019-1183-5

At a glance: The STEP trials

Springer Medicine

Many continuous variables should be analyzed using the relative scale: a case study of β₂-agonists for preventing exercise-induced bronchoconstriction

Abstract

Background

Methods

Results

Conclusions

At a glance: The STEP trials

Springer Medicine

Abstract

Background

Methods

Results

Conclusions

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