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Evaluation Review, Vol. 29, No. 3, 241-267 (2005)
DOI: 10.1177/0193841X04273257
© 2005 SAGE Publications

Increasing the Degrees of Freedom in Existing Group Randomized Trials

The df* Approach

Jonathan L. Blitstein

RTI International

Peter J. Hannan

University of Minnesota

David M. Murray

University of Memphis

William R. Shadish

University of California, Merced

This study describes a method for incorporating external estimates of intraclass correlation to improve the precision for the analysis of an existing group-randomized trial. The authors use a random-effects meta-analytic approach to pool the information across studies, which takes into account any interstudy heterogeneity that may exist. This approach can be used in several different situations to estimate the degrees of freedom available for an adjusted test of the intervention effect in a study where the challenges of group-randomized trials were not fully considered when the study was planned. The authors discuss the limitations of this approach and the circumstances in which it is likely to be helpful.

Key Words: group-randomized trial • degrees of freedom • power • meta-analysis


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