International Journal of Epidemiology, Vol 26, 765-771, Copyright © 1997 by International Epidemiological Association
DJ Slymen and MF Hovell
BACKGROUND: The decision to randomize by clusters of subjects such as a
classroom or clinic versus individual randomization where some
contamination may occur is examined within the framework of sample size
issues. Estimates for background rates and intraclass correlations are also
provided for adolescent tobacco and alcohol outcomes derived from a recent
study using cluster randomization. METHODS: A ratio of adjusted sample
sizes is derived which is a function of the intraclass correlation and
cluster size for cluster randomization and total amount of contamination
for individual randomization. Using estimated incidence rates and
intraclass correlations, we provide a comparison of sample sizes for two
plausible study outcomes. RESULTS: Small clusters such as a family or small
classroom tend to have stronger within cluster dependence and cluster
randomization would be clearly favoured over individual randomization. For
moderately sized clusters, if contamination levels are likely to be high
then cluster randomization would be a better choice. However in some
situations where lower levels of contamination are expected, individual
randomization may be preferred. With larger clusters, individual
randomization should be considered when contamination rates are expected to
be low. CONCLUSIONS: Investigators must carefully consider the choice of
cluster randomization versus individual randomization in the context of
likely contamination. In this paper we provided a basis for making this
decision as well as examples to illustrate these decisions, and parameter
estimates that will be especially useful for investigators in adolescent
tobacco and alcohol studies.
ARTICLES
Cluster versus individual randomization in adolescent tobacco and alcohol studies: illustrations for design decisions
Center for Behavioral Epidemiology, Graduate School of Public Health, San Diego State University, CA 92182-4162, USA.
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