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Trials
Published in: Trials 1/2022

Open Access 01-12-2022 | Methodology

Planning a method for covariate adjustment in individually randomised trials: a practical guide

Authors: Tim P. Morris, A. Sarah Walker, Elizabeth J. Williamson, Ian R. White

Published in: Trials | Issue 1/2022

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Abstract

Background

It has long been advised to account for baseline covariates in the analysis of confirmatory randomised trials, with the main statistical justifications being that this increases power and, when a randomisation scheme balanced covariates, permits a valid estimate of experimental error. There are various methods available to account for covariates but it is not clear how to choose among them.

Methods

Taking the perspective of writing a statistical analysis plan, we consider how to choose between the three most promising broad approaches: direct adjustment, standardisation and inverse-probability-of-treatment weighting.

Results

The three approaches are similar in being asymptotically efficient, in losing efficiency with mis-specified covariate functions and in handling designed balance. If a marginal estimand is targeted (for example, a risk difference or survival difference), then direct adjustment should be avoided because it involves fitting non-standard models that are subject to convergence issues. Convergence is most likely with IPTW. Robust standard errors used by IPTW are anti-conservative at small sample sizes. All approaches can use similar methods to handle missing covariate data. With missing outcome data, each method has its own way to estimate a treatment effect in the all-randomised population. We illustrate some issues in a reanalysis of GetTested, a randomised trial designed to assess the effectiveness of an electonic sexually transmitted infection testing and results service.

Conclusions

No single approach is always best: the choice will depend on the trial context. We encourage trialists to consider all three methods more routinely.
Appendix
Available only for authorised users
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Metadata
Title
Planning a method for covariate adjustment in individually randomised trials: a practical guide
Authors
Tim P. Morris
A. Sarah Walker
Elizabeth J. Williamson
Ian R. White
Publication date
01-12-2022
Publisher
BioMed Central
Published in
Trials / Issue 1/2022
Electronic ISSN: 1745-6215
DOI
https://doi.org/10.1186/s13063-022-06097-z

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