title: Planning a cluster randomized controlled trial: methodological issues.
abstract: 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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Nursing and Midwifery, Queen's University Belfast, 97 Lisburn Road, Belfast, Northern Ireland, United Kingdom. j.christie@qub.ac.uk
authors.forename: Janice
authors.initials: J
authors.lastname: Christie
authors.forename: Peter
authors.initials: P
authors.lastname: O'Halloran
authors.forename: Mike
authors.initials: M
authors.lastname: Stevenson
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D015982:Bias; D016000:Cluster Analysis; D003625:Data Collection / Q000379:methods; D003627:Data Interpretation, Statistical; D003663:Decision Trees; D006801:Humans; D015233:Models, Statistical; D015399:Nursing Research / Q000941:ethics / Q000379:methods*; D017063:Outcome Assessment, Health Care; D010934:Planning Techniques; D016032:Randomized Controlled Trials as Topic / Q000941:ethics / Q000379:methods* / Q000451:nursing; D012107:Research Design; D018401:Sample Size