Box 2.

: Three reasons for correlation of individual responses within area or organisational clusters

• Healthy subjects or patients may have chosen the social unit to which they belong. For example, individuals may select their general practitioners on the basis of characteristics such as age, sex, or ethnic group. Individuals who choose the same social or organisational unit might be expected to have something in common
• Cluster level attributes may have a common influence over all individuals in that cluster, thus making them more similar. For example, outcomes of surgery may vary systematically between surgeons, so that outcomes for patients treated by one surgeon tend to be more similar to each other than to those of another surgeon
• Individuals may interact within the cluster, leading to similarities between individuals for some health related outcomes. This might occur, for example, when individuals within a community respond to health promotion messages communicated through news media