Bias in identifying and recruiting participants in cluster randomised trials: what can be done?

(Summary Points) . Poor design of cluster trials risks bias in selection of participants. . Ideally participants should be identified before the cluster is randomised. . When this is not possible recruitment should be by someone masked to the cluster allocation. . Statistical solutions to selection bias are less satisfactory than design solutions. . Cluster trials need to report cluster sizes to enable the readers to ascertain any differences in recruitment between treatment groups.