Box 1.

: Reasons for carrying out evaluations at cluster level

• Public health and healthcare programmes are generally implemented at organisation rather than individual level, so cluster level studies are more appropriate for assessing the effectiveness of such programmes
• It may not be appropriate, or possible in practice, to randomise individuals to intervention groups since all individuals within a general practice or clinic may be treated in the same way
• “Contamination” may sometimes be minimised through allocation of appropriate organisational clusters to intervention and control groups. For example, individuals in an intervention group might communicate a health promotion message to control individuals in the same cluster. This might be minimised by randomising whole towns to different interventions
• Studies in which entire clusters are allocated to groups may sometimes be more cost effective than individual level allocation, if locating and randomising individuals is relatively costly