Increasing the degrees of freedom in future group randomized trials: The df* approach

Jonathan L. Blitstein, David M. Murray, Peter J. Hannan, William R. Shadish

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)268-286
Number of pages19
JournalEvaluation Review
Issue number3
StatePublished - Jun 1 2005


  • Causal inference
  • Degrees of freedom
  • Group-randomized trial
  • Power

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