The association between an individual's particular characteristics and the context within which s/he lives has been the subject of research for social and health scientists for some time.1-3 This work has had an impact on research in primary care, with the increasing use of cluster randomised controlled trials throughout the world. Recent examples have included a study evaluating the effects of injury prevention training on the knowledge, attitudes and practices of midwives and health visitors in the UK4; a study that assessed the long-term effectiveness of the ‘green prescription’ on the health of 40–79 year-old patients in New Zealand5; and a study evaluating the effectiveness of a multi-factorial intervention in reducing falls in older people in residential homes in Sweden.6
The essential nature of health service organisation has meant that the use of these trials has become common, replacing the more traditional trials where participants are individually randomised. Proponents of cluster randomised controlled trials argue that in ‘real life’ people are socially grouped in a way that their individual characteristics may be linked to being part of that group. In primary care research these ‘clusters’ may be clinics or general practices, for example. By ignoring the contribution of these groupings on people's knowledge, attitudes and behaviours in designing and analysing studies, an aspect of potential bias is ignored.
Trial design issues
There are some issues that are pertinent particularly to cluster randomised trials. For example, as a result of the loss of power due to the variability between clusters, sample sizes need to be larger than in individually randomised trials. Estimates are needed, usually from previous studies of the ‘intra-cluster correlation coefficient’. If this ‘design …