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Evaluation Review, Vol. 29, No. 3, 268-286 (2005)
DOI: 10.1177/0193841X04273258
© 2005 SAGE Publications

Increasing the Degrees of Freedom in Future Group Randomized Trials

The df* Approach

Jonathan L. Blitstein

RTI International

David M. Murray

University of Memphis

Peter J. Hannan

University of Minnesota

William R. Shadish

University of California, Merced

This article builds on the previous article by Blitstein et al. (2005), which showed how external estimates of intraclass correlation can be used to improve the precision for the analysis of an existing group randomized trial. The authors extend that work to sample size estimation and power analysis for future group-randomized trials. Often this approach will allow a smaller study than would otherwise be possible without sacrificing statistical power. Such studies are needed, for example, as pilot studies to help plan for a full-scale efficacy trial, as replication studies, or in situations in which resource constraints prohibit a larger trial. The authors discuss the circumstances under which this strategy will be most helpful and the risks associated with conducting smaller studies.

Key Words: group-randomized trial • degrees of freedom • power • causal inference


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