The unit of allocation in most clinical trials is the individual patient. However, experimental trials in which the unit of allocation is an intact cluster of participants (eg, families, schools, medical practices, communities) are becoming increasingly widespread in the evaluation of healthcare and educational interventions. For example, Avorn et al describe the results of a health education trial (aimed at staff personnel in nursing homes) that was designed to reduce the use of psychoactive drugs by residents.1 Six pair matched facilities were included in this trial, with 1 facility in each matched pair randomly assigned to the educational programme and the other facility serving as a control. Several reasons exist for favouring cluster randomisation in this trial. A principal one would be to avoid the experimental contamination which could occur when the same personnel are asked to give both interventions to different participants and when knowledge of the intervention may influence the responses of participants in the control group. A second reason is that the assignment of a new educational programme to some individuals within a nursing home but not to others might be regarded as unacceptable, or even unethical, by some practitioners. Finally, having administratively set up such a programme within a facility, it would seem much more likely to function effectively from a practical perspective if all staff members, and not just some, were involved. A notable design feature of this trial is that the 12 nursing homes recruited were pair matched on the basis of size, type of ownership, and level of drug use. The purpose of such matching was to ensure that the facilities in each pair were similar with respect to baseline drug use, but geographically distant enough to minimise the risk of experimental contamination that could arise through the sharing of knowledge. Such …