Methods in health service research - Evaluation of health interventions at area and organisation level

Health interventions are often implemented at the levels of health service organisational unit or of geographical or administrative area. The unit of intervention is then a cluster of individual patients or healthy subjects. Evaluation of cluster level interventions may be difficult because only a few units of large size may be available for study evaluation may be at either individual or cluster level, and individuals' responses may be correlated within clusters. At the design stage, it is important to randomise clusters whenever possble, adapt sample size calculations to allow for clustering of responses, and choose between cohort and repeated cross sectional designs. Methods chosen for analysis of individual data should take into account tile correlation of individual responses within clusters.