[bull ] Many interventions in health and health care are implemented at the level of geographic area or health service organizational unit. Such interventions will be implemented for entire clusters of healthy subjects or patients.

[bull ] Evaluation of cluster level interventions may be difficult because only a small number of clusters may be available for study and the responses of individuals are correlated within clusters.

[bull ] This review synthesized current methodologic recommendations into a checklist for investigators:

1. Recognize the cluster as the unit of intervention or allocation;

2. Justify the use of the cluster as the unit of intervention or allocation;

3. Include a sufficient number of clusters;

4. Randomize clusters wherever possible;

5. In nonrandomized studies include a control group;

6. In single group studies include repeated measurements over time;

7. Allow for clustering when estimating the required sample size;

8. Consider matching or stratifying clusters where appropriate;

9. Choose between repeated cross-sectional and cohort designs in studies with follow-up;

10. Allow for clustering at the time of analysis;

11. Allow for confounding at both individual and cluster level; and

12. Include estimates of intraclass correlation and components of variance in published reports.

[bull ] The present level of adherence to these recommendations is low.