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Allan Donner, PhD and Neil Klar, PhD

Allan Donner is with the Department of Epidemiology and Biostatistics, University of Western Ontario, London. Neil Klar is with the Division of Preventive Oncology, Cancer Care Ontario, Toronto.

Correspondence: Requests for reprints should be sent to Allan Donner, PhD, Department of Epidemiology and Biostatistics, University of Western Ontario, Room K201, Kresge Building, London, Ontario, Canada N6A 5C1 (e-mail: donner{at}biostats.uwo.ca).

It is now well known that standard statistical procedures become invalidated when applied to cluster randomized trials in which the unit of inference is the individual. A resulting consequence is that researchers conducting such trials are faced with a multitude of design choices, including selection of the primary unit of inference, the degree to which clusters should be matched or stratified by prognostic factors at baseline, and decisions related to cluster subsampling. Moreover, application of ethical principles developed for individually randomized trials may also require modification.

We discuss several topics related to these issues, with emphasis on the choices that must be made in the planning stages of a trial and on some potential pitfalls to be avoided.

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