A protocol for a systematic review on the impact of unpublished studies and studies published in the gray literature in meta-analyses

Christine Schmucker, Annette Bluemle, Matthias Briel, Susan Portalupi, Britta Lang, Edith Motschall, Guido Schwarzer, Dirk Bassler, Katharina F Mueller, Erik von Elm, Joerg J Meerpohl

Meta-analyses are particularly vulnerable to the effects of publication bias. Despite methodologists' best efforts to locate all evidence for a given topic the most comprehensive searches are likely to miss unpublished studies and studies that are published in the gray literature only. If the results of the missing studies differ systematically from the published ones, a meta-analysis will be biased with an inaccurate assessment of the intervention's effects.As part of the OPEN project (http://www.open-project.eu) we will conduct a systematic review with the following objectives:▪ To assess the impact of studies that are not published or published in the gray literature on pooled effect estimates in meta-analyses (quantitative measure).▪ To assess whether the inclusion of unpublished studies or studies published in the gray literature leads to different conclusions in meta-analyses (qualitative measure).