This is an archived version of the Handbook. For the current version, please go to training.cochrane.org/handbook/current.

While publication bias has long been recognized and much discussed, other factors can contribute to biased inclusion of studies in meta-analyses. Indeed, among published studies, the probability of identifying relevant studies for meta-analysis is also influenced by their results. These biases have received much less consideration than publication bias, but their consequences could be of equal importance.