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Planning a Cluster Randomized Controlled Trial: Methodological Issues

Christie, Janice; O'Halloran, Peter; Stevenson, Mike

doi: 10.1097/NNR.0b013e3181900cb5

Background: The standard approach in a randomized controlled trial (RCT) is to randomize individuals to intervention and control groups. Yet, nursing and other health interventions are often implemented at the levels of health service organizational unit or geographical area. It may be more appropriate to conduct a cluster RCT. However, cluster randomization requires consideration of a number of important issues.

Objective: The objective of this study was to show how critical issues in relation to design and analysis can be addressed.

Approach: Two cluster RCTs conducted by the authors are used as examples. Guidance on the conduct and reporting of cluster RCTs is also offered.

Results: A rationale for choosing this design was provided, and issues in relation to study design, calculation of sample size, and statistical analysis were clarified. A decision tree and checklist are provided to guide researchers through essential steps in conducting a cluster RCT.

Discussion: Cluster RCTs present special challenges in relation to design, conduct, and analysis. Nevertheless, they are an appropriate and potentially powerful tool for nursing research. With careful attention to the issues addressed in this article, researchers can use this approach successfully.

Janice Christie, PhD, MA, RN, RSCPHN, PGCE, is Teaching Fellow, School of Nursing and Midwifery; Peter O'Halloran, PhD, MSc, RN, RNT, is Lecturer, School of Nursing and Midwifery; and Mike Stevenson, BSc, FSS, PGCHET, is Senior Lecturer in Medical Statistics, School of Medicine and Dentistry, Queen's University Belfast, Belfast, Northern Ireland, United Kingdom.

Editor's Note Additional information provided by the authors expanding this article is on the Editor's Web site at

Accepted for publication July 10, 2008.

The authors thank Professor Jean Orr and Mrs. Marianne Moutray, head and associate head of the School of Nursing and Midwifery, Queen's University Belfast, respectively, for their help and support in undertaking this article. Acknowledgement is also due to the Research and Development Office, Department of Health, Social Services and Public Safety in Northern Ireland which supported and funded the original research upon which this article is based.

Corresponding author: Janice Christie, PhD, MA, RN, RSCPHN, PGCE, School of Nursing and Midwifery, MBC Building, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, United Kingdom (e-mail:

© 2009 Lippincott Williams & Wilkins, Inc.