Additional material is published online only. To view please visit the journal online. Timeline cluster: a graphical tool to identify risk of bias in cluster randomised trials Agnès Caille, 1 ,2 ,3 Sally Kerry, 4 Elsa Tavernier, 2 ,3 Clémence Leyrat, 5 Sandra Eldridge, 4 Bruno Giraudeau 1 ,2 ,3 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.In cluster randomised trials, clusters of subjects such as hospitals or family practices are randomised rather than people themselves. 1 Cluster randomised trials are used for evaluating health service organisation and health policy, often with complex interventions targeted at the level of the cluster, the individual, or both. Randomisation should prevent allocation bias at the cluster level provided that it is properly conducted, but differences in individual level characteristics between the intervention arms can be reintroduced because of the relative timing of participant identification and recruitment, and cluster randomisation. Indeed, the usual chronology of an individual randomised trial with first recruitment and then randomisation of participants can be reversed in cluster randomised trials: the identification and recruitment of participants often take place after randomisation, which could lead to identification or recruitment bias (hereafter called recruitment bias). 2 3 Because blinding is rarely possible for interventions assessed in cluster randomised trials, previous knowledge of the allocation from recruiters or participants can influence who is approached and who agrees to participate in a trial. This might lead to different recruitment rates between arms as well as imbalance in participant characteristics. [4][5][6] Some solutions proposed to prevent recruitment bias include the identification and recruitment of participants before cluster randomisation or recruitment of participants by a blinded and independent person. 7 These solutions should be considered whenever possible, but they are not always feasible or applied. Furthermore, cluster randomised trials are prone to other biases, usually encountered when blinding is lacking-n...