The source of search for literature in meta-analysis includes the published literature, unpublished literature, uncompleted research reports, and work in progress. Reliance on only published reports leads to publication bias-the bias resulting from the tendency to publish results that are statistically significant. Hence, the meta-analysis needs to obtain information from unpublished research also. Contact with colleagues, experts in the applied field, and other informed channels can be important sources of information on published, uncompleted, and ongoing studies.