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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
medicine
Analyses of 'change scores' do not estimate causal effects in observational data
International Journal of Epidemiology, Volume 51, No. 5, Year 2022
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Description
Background: In longitudinal data, it is common to create 'change scores' by subtracting measurements taken at baseline from those taken at follow-up, and then to analyse the resulting 'change' as the outcome variable. In observational data, this approach can produce misleading causal-effect estimates. The present article uses directed acyclic graphs (DAGs) and simple simulations to provide an accessible explanation for why change scores do not estimate causal effects in observational data. Methods: Data were simulated to match three general scenarios in which the outcome variable at baseline was a (i) 'competing exposure' (i.e. a cause of the outcome that is neither caused by nor causes the exposure), (ii) confounder or (iii) mediator for the total causal effect of the exposure variable at baseline on the outcome variable at follow-up. Regression coefficients were compared between change-score analyses and the appropriate estimator(s) for the total and/or direct causal effect(s). Results: Change-score analyses do not provide meaningful causal-effect estimates unless the baseline outcome variable is a 'competing exposure' for the effect of the exposure on the outcome at follow-up. Where the baseline outcome is a confounder or mediator, change-score analyses evaluate obscure estimands, which may diverge substantially in magnitude and direction from the total and direct causal effects. Conclusion: Future observational studies that seek causal-effect estimates should avoid analysing change scores and adopt alternative analytical strategies. © 2021 The Author(s). Published by Oxford University Press on behalf of the International Epidemiological Association.
Authors & Co-Authors
Ellison, George T.H.
United Kingdom, Leeds
University of Leeds
United Kingdom, Preston
University of Central Lancashire
Gilthorpe, Mark S.
United Kingdom, Leeds
University of Leeds
United Kingdom, London
The British Library
Statistics
Citations: 71
Authors: 2
Affiliations: 3
Identifiers
Doi:
10.1093/ije/dyab050
ISSN:
03005771
Study Design
Cohort Study