| Summary: | This thesis examines aspects of the application of the cluster randomised trial design in implementation research. In particular, it focuses on three main themes: a) the intracluster correlation coefficient (ICC) in cluster randomised trials of implementation research and factors affecting its magnitude; b) the impact of the ICC on sample size calculations; and c) reporting considerations for cluster randomised trials generally, and for ICCs in particular. The results show that empirical estimates of ICCs vary in size and certain factors - particularly the type of variable and the study setting - influence the magnitude of the ICC. When reporting an ICC, three aspects were found to be important: a) a description of the dataset, b) information on how the ICC was calculated; and c) information on the accuracy of the ICC. General reporting considerations for cluster randomised trials were also considered and found to require explanations for: a) the rationale for adopting a cluster design; b) how the effects of clustering were incorporated into the sample size calculations; c) how the effects of clustering were incorporated into the analysis; and d) the flow of both clusters and individuals through the trial, from assignment to analysis. A revised CONSORT statement accommodating the specific features of a cluster randomised trial was presented. The work outlined in this thesis shows that the adoption of a cluster randomised trial in implementation research has a number of important implications that researchers should consider when planning future trials.
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