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Related Experiment Video

Updated: Nov 6, 2025

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Contamination: How much can an individually randomized trial tolerate?

Karla Hemming¹, Monica Taljaard², Mirjam Moerbeek³

¹Institute of Applied Health Research, University of Birmingham, Birmingham, UK.

Statistics in Medicine

|May 7, 2021

Summary

Individual randomization may still be preferred over cluster randomization, even with contamination. Individually randomized trials can tolerate significant contamination before cluster designs become more efficient.

Keywords:

cluster-randomized trials contamination individually randomized trials statistical efficiency

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Area of Science:

Clinical Trial Design
Biostatistics
Epidemiology

Background:

Cluster randomization often requires larger sample sizes than individual randomization due to efficiency loss.
Contamination, where treatment effects spill over between participants, complicates sample size calculations in individually randomized trials.
Existing comparisons typically assume no contamination in individually randomized trials.

Purpose of the Study:

To develop a framework for determining the tolerable level of contamination in individually randomized trials before cluster randomization becomes more sample-size efficient.
To compare sample size requirements between individually randomized and cluster randomized designs under varying degrees of contamination.

Main Methods:

Developed a general framework to calculate the critical contamination rate.
Analyzed various cluster trial designs: parallel-arm, stepped-wedge, and cluster crossover.
Calculated sample size needs for individually randomized trials detecting attenuated effects versus cluster randomized trials with non-attenuated effects.

Main Results:

Individually randomized trials can tolerate substantial contamination before cluster designs require a larger sample size.
The critical contamination rate is a function of design effects for clustering, multiple periods, stratification, and repeated measures.
Cluster randomized designs carry a higher risk of unpredictable bias due to differential recruitment without blinding.

Conclusions:

Despite contamination risks, individual randomization may remain the preferred design choice due to its lower risk of unpredictable bias compared to cluster designs.
The findings are crucial for pragmatic trial comparisons, especially when contamination predictably attenuates treatment effects.