The preceding domains (sequence generation, allocation concealment, blinding, incomplete outcome data and selective outcome reporting) relate to important potential sources of bias in clinical trials across all healthcare areas.  Beyond these specific domains, however, review authors should be alert for further issues that may raise concerns about the possibility of bias. This seventh domain in the ‘Risk of bias’ assessment tool is a ‘catch-all’ for other such sources of bias. For reviews in some topic areas, there may be additional questions that should be asked of all studies. In particular, some study designs warrant special consideration when they are encountered. If particular study designs are anticipated (e.g. cross-over trials, or types of non-randomized study), additional questions relating to the risk of bias in these types of studies may be posed. Assessing risk of bias in non-randomized studies is addressed in Chapter 13, and risk of bias for cluster-randomized trials, cross-over trials and trials with multiple intervention groups is addressed in Chapter 16. Furthermore, some major, unanticipated, problems with specific studies may be identified during the course of the systematic review or meta-analysis. For example, a trial may have substantial imbalance of participant characteristics at baseline. Several examples are discussed in the sections that follow.