Bias in meta-analytic research.

With the proliferation of meta-analyses in the medical literature have come conflicting studies. In addition, observance of guidelines for the performance of meta-analyses has been spotty. Bias may explain conflicting studies and differentiate carefully performed meta-analyses from others. Meta-analysts may fail to anticipate biases which threaten their study's validity. The three stages at which bias can be injected into a meta-analysis are finding studies, selection of the identified studies for the meta-analysis and extraction of data from the selected studies. This manuscript reviews specific types of bias which are common at each of these stages.