This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Design-specific risks of bias

The principal concern over risk of bias in non-randomized studies is selection bias in the form of differences in types of participants between experimental and control intervention groups. Review authors should refer to the full discussion in Chapter 13 (Section 13.5). The main concerns over risk of bias in cluster-randomized trials are: (i) recruitment bias (differential participant recruitment in clusters for different interventions); (ii) baseline imbalance; (iii) loss of clusters; (iv) incorrect analysis; and (v) comparability with individually randomized trials. The main concerns over risk of bias in cross-over trials are: (i) whether the cross-over design is suitable; (ii) whether there is a carry-over effect; (iii) whether only first period data are available; (iv) incorrect analysis; and (v) comparability of results with those from parallel-group trials. These are discussed in detail in Chapter 16 (Sections 16.3 and 16.4). Risk of bias in studies with more than two intervention groups is also discussed in Chapter 16 (Section 16.5).