Timeline cluster: a graphical tool to identify risk of bias in cluster randomised trials

Robust evidence of the effectiveness of interventions relating to policy, practice, and organisation of healthcare often comes from well conducted cluster randomised trials. Such trials are, however, prone to recruitment bias depending on whether participants are recruited before the randomisation of clusters and whether the recruiter is blinded to the allocation status. In most cases, participants and trial staff cannot be blinded to the intervention, which might lead to performance and detection bias. Unfortunately, cluster trial reports often do not provide a clear description of the timing of trial processes and blinding, and these aspects are not covered by current reporting tools. This article proposes a graphical tool depicting the time sequence of steps and blinding status in cluster randomised trials. The tool might be helpful at both the protocol and the report writing stages to clarify the process and to help identify potential bias in cluster randomised trials.