Determinants of the intracluster correlation coefficient in cluster randomized trials: the case of implementation research

Research output: Contribution to journalArticlepeer-review

191 Citations (Scopus)


The objective of this research was to identify determinants of the magnitude of intracluster correlation coefficients (ICCs) in cluster randomized trials from the field of implementation research. A survey of experts was conducted to generate a priori hypotheses of factors that might affect ICC size. Hypotheses were tested on empirical estimates of ICCs calculated from 21 implementation research datasets, mainly from the UK. Effects of setting (primary or secondary care), type of variable (process or outcome), type of measurement (objective or subjective), prevalence of outcome and size of cluster were tested. In total, 220 ICCs were available (range 0 to 0.415). Significant differences in ICC magnitude were found. The ICCs were significantly higher for process than for outcome variables, and for secondary care outcomes compared with primary care outcomes. The effects of prevalence and size were less clear cut. There was no evidence to suggest that type of measurement affected ICC size. In conclusion, accurate estimates of ICCs are essential for sample size calculations for cluster randomized trials of professional behaviour change interventions. This study demonstrates that ICCs are sensitive to a number of trial factors, particularly setting and outcome type. These factors must be considered when planning such cluster randomized trials.
Original languageEnglish
Pages (from-to)99-107
Number of pages9
JournalClinical Trials
Issue number2
Publication statusPublished - 1 Apr 2005

Bibliographical note

The Health Services Research Unit is funded by the Chief Scientist Office of the Scottish Executive Health Department. JMG holds a Canada Research Chair in Health Knowledge Transfer and Uptake. The views expressed are not necessarily those of the funding body. We thank the referees for their helpful and insightful comments.


Dive into the research topics of 'Determinants of the intracluster correlation coefficient in cluster randomized trials: the case of implementation research'. Together they form a unique fingerprint.

Cite this