Methods for obtaining unpublished data

Background In order to minimise publication bias, authors of systematic reviews often spend considerable time trying to obtain unpublished data. These include data from studies conducted but not published (unpublished data), as either an or full‐text paper, as well as missing data (data available to original researchers but not reported) in published s or full‐text publications. The effectiveness of different methods used to obtain unpublished or missing data has not been systematically evaluated. Objectives To assess the effects of different methods for obtaining unpublished studies (data) and missing data from studies to be included in systematic reviews. Search methods We identified primary studies comparing different methods of obtaining unpublished studies (data) or missing data by searching the Cochrane Methodology Register (Issue 1, 2010), MEDLINE and EMBASE (1980 to 28 April 2010). We also checked references in relevant reports and contacted researchers who were known or who were thought likely to have carried out relevant studies. We used the Science Citation Index and PubMed 'related articles' feature to identify any additional studies identified by other sources (19 June 2009). Selection criteria Primary studies comparing different methods of obtaining unpublished studies (data) or missing data in the healthcare setting. Data collection and analysis The primary outcome measure was the proportion of unpublished studies (data) or missing data obtained, as defined and reported by the authors of the included studies. Two authors independently assessed the search results, extracted data and assessed risk of bias using a standardised data extraction form. We resolved any disagreements by discussion. Main results Six studies met the inclusion criteria; two were randomised studies and four were observational comparative studies evaluating different methods for obtaining missing data. Methods to obtain missing data Five studies, two randomised studies and three observational comparative studies, assessed methods for obtaining missing data (i.e. data available to the original researchers but not reported in the published study). Two studies found that correspondence with study authors by e‐mail resulted in the greatest response rate with the fewest attempts and shortest time to respond. The difference between the effect of a single request for missing information (by e‐mail or surface mail) versus a multistage approach (pre‐notification, request for missi