Including variants on randomized trials

In cluster-randomized trials, groups of individuals rather than individuals are randomized to different interventions. For the purposes of an assessment of risk of bias using the RoB 2 tool, this chapter defines participants in cluster-randomized trials as those on whom investigators seek to measure the outcome of interest. Parallel-group trials allocate each participant to a single intervention for comparison with one or more alternative interventions. In contrast, crossover trials allocate each participant to a sequence of interventions. Crossover trials are suitable for evaluating interventions with a temporary effect in the treatment of stable, chronic conditions. To include a study with more than two intervention groups in a meta-analysis, a recommended approach is to omit groups that are not relevant to the comparison being made, and to combine multiple groups that are eligible as the experimental or comparator intervention to create a single pair-wise comparison. Alternatively, multi-arm studies are dealt with appropriately by network meta-analysis.