Cited by 99Statistical Methods in Clinical TrialsAdvanced Causal Inference TechniquesStatistical Methods and Bayesian Inference

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

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Morris, Tim P., Walker, A. Sarah, Williamson, Elizabeth J., White, Ian R.·2022·Trials

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.

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  • Adjusting for baseline covariates, including those used in stratified randomisation, can lead to improved power to detect an intervention effect, particularly when a baseline covariate is prognostic of the outcome.

    “Adjusting for baseline covariates, including those used in any stratified randomisation, is often advised in the analysis, particularly when a baseline covariate is prognostic of the outcome, as it can lead to improved power to detect an intervention effect.408 409 410”

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Authors
Morris, T. P., Walker, A. S., Williamson, E. J., & White, I. R.
Affiliations
  • London School of Hygiene & Tropical Medicine (GB)
  • MRC Clinical Trials Unit at UCL (GB)
Corresponding author
Morris, Tim P.
Venue
Trials
Published
2022-04-18
Volume
23
Issue
1
Pages
328-328
ISSN
1745-6215 (ISSN-L)
OpenAlex
W3180549268
Cited by
97
References
47
Citations by year
2026: 17 · 2025: 28 · 2024: 29 · 2023: 17 · 2022: 5 · 2021: 1
FWCI
15.12
Type
journal article
Language
en
Open access
Locations
  • Trials (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • LSHTM Research Online (London School of Hygiene and Tropical Medicine) (acceptedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • UCL Discovery (University College London) (submittedVersion)
Funders
medical research council (MC_UU_12023/21, MC_UU_12023/29, MR/L004933/2/N96, MR/S01442X/1)
Topics
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
  • Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods and Bayesian Inference
Keywords
covariate adjustment, baseline covariates, statistical analysis plan, standardization, inverse probability of treatment weighting, robust standard errors, missing covariates, missing outcome data, treatment effect estimation
MeSH terms
Humans, Probability, Research Design, Sample Size

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