The sample size justification for a cluster randomized trial is messy. It requires the use of an intra-class correlation or something similar (the authors use the term within-cluster correlation). In a review of 300 cluster randomized trials
- the authors found that in only about a third of the trials did the authors specify the within-cluster correlation. Even fewer compared this to the observed within-cluster correlation observed in the data. We need to do better.
- Taljaard M
- Dixon S
- Copas A
- Eldridge S. Reporting and methodological quality of sample size calculations in cluster randomized trials could be improved: a review. Journal of Clinical Epidemiology 68 (2015) 716e723. Available at http://www.ncbi.nlm.nih.gov/pubmed/25523375.