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This is the second of four articles Healthcare interventions are often implemented at the level of the organisation or geographical area rather than at the level of the individual patient or healthy subject. For example, screening programmes are delivered to residents of a particular area; health promotion interventions might be delivered to towns or schools; general practitioners deliver services to general practice populations; hospital specialists deliver health care to clinic populations. Interventions at area or organisation level are delivered to clusters of individuals. The evaluation of interventions based in an area or organisation may require the allocation of clusters of individuals to different intervention groups (see box 1).1 2 Cluster based evaluations present special problems both in design and analysis.3 Often only a small number of organisational units of large size are available for study, and the investigator needs to consider the most effective way of designing a study with this constraint. Outcomes may be evaluated either at cluster level or at individual level (table).4 Often cluster level interventions are aimed at modifying the outcomes of the individuals within clusters, and it will then be important to recognise that outcomes for individuals within the same organisation may tend to be more similar than for individuals in different organisational clusters (see box 2). This dependence between individuals in the same cluster has important implications for the design and analysis of organisation based studies.2 This paper addresses these issues. #### Summary points 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 subject 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 …