OBJECTIVES: To describe the health care settings, purposes, and study reporting quality of the 2 × 2 cluster randomized controlled factorial trial design.
STUDY DESIGN AND SETTING: This study is a systematic review. We searched Medline, Embase, Cochrane Library, and Web of Knowledge for articles published up to May 2012. Cluster randomized controlled 2 × 2 factorial trials in health, evaluating at least one complex intervention, were included. Two authors independently reviewed and extracted data from the studies.
RESULTS: Twenty-nine studies covering a wide range of clinical areas and health care settings were included. The cluster design was mostly used to minimize contamination. The factorial design was mostly used to assess the effects of two interventions in the same study and to explore interactions between interventions. However, although most studies explored the presence or absence of intervention interactions, they were often either not sufficiently powered to detect any interactions or did not provide information on whether the study was sufficiently powered to detect any interactions. There was a considerable variability in the reporting of a number of study characteristics and methodological aspects. Study quality was also variable within and across studies.
CONCLUSION: The design has been used in a wide range of health care settings and clinical areas to minimize contamination, assess the effects of two interventions in the same study, and explore intervention interactions. There is need for improvement on and guidelines for the reporting of factorial trials.