Methods for obtaining unpublished data.

In order to minimise publication bias, authors of systematic reviews often spend considerable time trying to obtain unpublished data. These include data from studies conducted but not published (unpublished data), as either an abstract or full-text paper, as well as missing data (data available to original researchers but not reported) in published abstracts or full-text publications. The effectiveness of different methods used to obtain unpublished or missing data has not been systematically evaluated.