title: A protocol for a systematic review on the impact of unpublished studies and studies published in the gray literature in meta-analyses.
abstract: BACKGROUND
  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best efforts to locate all evidence for a given topic the most comprehensive searches are likely to miss unpublished studies and studies that are published in the gray literature only.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gray literature on pooled effect estimates in meta-analyses (quantitative measure).▪ To assess whether the inclusion of unpublished studies or studies published in the gray literature leads to different conclusions in meta-analyses (qualitative measure).
  
  
  METHODS
  
  
  
  METHODS
  Methodological research projects of a cohort of meta-analyses which compare the effect of the inclusion or exclusion of unpublished studies or studies published in the gray literature.
  
  
  METHODS
  To identify relevant research projects we will conduct electronic searches in Medline, Embase and The Cochrane Library; check reference lists; and contact experts.
  
  
  RESULTS
  1) The extent to which the effect estimate in a meta-analyses changes with the inclusion or exclusion of studies that were not published or published in the gray literature; and 2) the extent to which the inclusion of unpublished studies impacts the meta-analyses' conclusions.
  
  
  METHODS
  Information will be collected on the area of health care; the number of meta-analyses included in the methodological research project; the number of studies included in the meta-analyses; the number of study participants; the number and type of unpublished studies; studies published in the gray literature and published studies; the sources used to retrieve studies that are unpublished, published in the gray literature, or commercially published; and the validity of the methodological research project.
  
  
  RESULTS
  DATA SYNTHESIS will involve descriptive and statistical summaries of the findings of the included methodological research projects.
  
  
  CONCLUSIONS
  Results are expected to be publicly available in the middle of 2013.
authors: German Cochrane Center, Institute of Medical Biometry and Medical Informatics, University Medical Center Freiburg, 79110 Freiburg, Germany.
authors: Christine
authors: C
authors: Schmucker
authors: Annette
authors: A
authors: Bluemle
authors: Matthias
authors: M
authors: Briel
authors: Susan
authors: S
authors: Portalupi
authors: Britta
authors: B
authors: Lang
authors: Edith
authors: E
authors: Motschall
authors: Guido
authors: G
authors: Schwarzer
authors: Dirk
authors: D
authors: Bassler
authors: Katharina F
authors: KF
authors: Mueller
authors: Erik
authors: E
authors: von Elm
authors: Joerg J
authors: JJ
authors: Meerpohl
mesh_terms: D022126:Access to Information; D015982:Bias; D006801:Humans; D033181:Information Dissemination; D015201:Meta-Analysis as Topic; D017594:Publication Bias; D012107:Research Design; D000078202:Systematic Reviews as Topic
references: Cochrane Database Syst Rev. 2007 Apr 18;(2):MR000010
references: 17443631
references: Syst Rev. 2013 Jan 09;2:2
references: 23302739
references: Health Technol Assess. 2000;4(10):1-115
references: 10932019
references: JAMA. 1993 Jun 2;269(21):2749-53
references: 8492400
references: Lancet. 2000 Oct 7;356(9237):1228-31
references: 11072941
references: BMJ. 2011 Jul 22;343:d4002
references: 21784880
references: BMJ. 2003 Sep 6;327(7414):557-60
references: 12958120