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A scoping review identified additional considerations for defining estimands in cluster randomized trials
Dongquan Bi1, Andrew Copas1, Fan Li2
1MRC Clinical Trials Unit at UCL, Institute of Clinical Trials and Methodology, University College London, London, UK.
Journal of Clinical Epidemiology
|October 12, 2025
Summary
Defining estimands for cluster randomized trials (CRTs) requires considering eight additional items beyond standard guidelines. These factors ensure treatment effect clarity for clinicians, patients, and policymakers in complex CRT designs.
Area of Science:
- Biostatistics
- Clinical Trials Methodology
- Epidemiology
Background:
- Estimands define the specific treatment effect a study quantifies, crucial for interpretable results.
- International Council for Harmonisation E9(R1) addendum provides five attributes for estimand definition, primarily for individually randomized trials.
- Cluster randomized trials (CRTs) introduce unique complexities in estimand definition due to group-level randomization.
Purpose of the Study:
- To identify additional considerations for defining estimands specifically within the context of cluster randomized trials (CRTs).
- To supplement the existing ICH E9(R1) guidelines with CRT-specific factors for robust estimand specification.
Main Methods:
- Systematic literature search across multiple databases and personal libraries.
- Identification of articles detailing estimand definition aspects not covered by the ICH E9(R1) addendum for CRTs.
- Synthesis of findings into a list of additional items for CRT estimand consideration.
Main Results:
- Eight key items were identified from 46 eligible articles for defining estimands in CRTs.
- These include: population of clusters, selection bias in individuals, exposure time, weighting methods (individual-average vs. cluster-average), marginal vs. cluster-specific measures, handling of cluster-level intercurrent events, interference/spillover, and participant attrition/movement between clusters.
- These items address complexities beyond those typically encountered in individually randomized trials.
Conclusions:
- This review highlights essential additional factors for estimand definition in CRTs.
- Investigators conducting CRTs should incorporate these eight items to ensure estimands are unambiguous and relevant.
- Clear estimands enhance the utility of CRT findings for end-users like clinicians, patients, and policymakers.
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