In most clinical trials participants are randomised as individuals to different treatments. Sometimes individual allocation is not possible or desirable, and groups of individuals are randomised instead: this is known as cluster or group randomisation. Many reasons for using cluster allocation exist
In most clinical trials participants are randomised as individuals to different treatments. Sometimes individual allocation is not possible or desirable, and groups of individuals are randomised instead: this is known as cluster or group randomisation. Many reasons for using cluster allocation exist. For example, evaluation of clinical guidelines or medical education on patient outcomes almost always requires that healthcare professionals are the “unit” of allocation.
Although randomised trials are the most robust evaluative method, poorly conducted studies are susceptible to different forms of selection bias that can make their results unsound. Methodological reviews of individually randomised trials have shown that rigorously conducted trials produce different effect estimates from poorly conducted studies.1,2
Less attention has been paid, however, to cluster trials. Cluster trials are generally more difficult to design and execute than individually randomised studies, and some design features of a cluster trial may make it especially vulnerable to a range of threats that can introduce selection bias.
In cluster trials potential bias in the execution of the trial can occur at two levels, the first of which is the cluster level. Randomisation of clusters needs to be undertaken carefully and preferably independently. Otherwise