title: Extent of non-publication in cohorts of studies approved by research ethics committees or included in trial registries.
abstract: BACKGROUND
  The synthesis of published research in systematic reviews is essential when providing evidence to inform clinical and health policy decision-making. However, the validity of systematic reviews is threatened if journal publications represent a biased selection of all studies that have been conducted (dissemination bias). To investigate the extent of dissemination bias we conducted a systematic review that determined the proportion of studies published as peer-reviewed journal articles and investigated factors associated with full publication in cohorts of studies (i) approved by research ethics committees (RECs) or (ii) included in trial registries.
  
  
  RESULTS
  Four bibliographic databases were searched for methodological research projects (MRPs) without limitations for publication year, language or study location. The searches were supplemented by handsearching the references of included MRPs. We estimated the proportion of studies published using prediction intervals (PI) and a random effects meta-analysis. Pooled odds ratios (OR) were used to express associations between study characteristics and journal publication. Seventeen MRPs (23 publications) evaluated cohorts of studies approved by RECs; the proportion of published studies had a PI between 22% and 72% and the weighted pooled proportion when combining estimates would be 46.2% (95% CI 40.2%-52.4%, I2 = 94.4%). Twenty-two MRPs (22 publications) evaluated cohorts of studies included in trial registries; the PI of the proportion published ranged from 13% to 90% and the weighted pooled proportion would be 54.2% (95% CI 42.0%-65.9%, I2 = 98.9%). REC-approved studies with statistically significant results (compared with those without statistically significant results) were more likely to be published (pooled OR 2.8; 95% CI 2.2-3.5). Phase-III trials were also more likely to be published than phase II trials (pooled OR 2.0; 95% CI 1.6-2.5). The probability of publication within two years after study completion ranged from 7% to 30%.
  
  
  CONCLUSIONS
  A substantial part of the studies approved by RECs or included in trial registries remains unpublished. Due to the large heterogeneity a prediction of the publication probability for a future study is very uncertain. Non-publication of research is not a random process, e.g., it is associated with the direction of study findings. Our findings suggest that the dissemination of research findings is biased.
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Schmucker
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Schell
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Susan
authors.initials: S
authors.lastname: Portalupi
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Oeller
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Laura
authors.initials: L
authors.lastname: Cabrera
authors.affiliation: Department of Neonatology, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Dirk
authors.initials: D
authors.lastname: Bassler
authors.affiliation: Institute of Medical Biometry and Statistics, Medical Center - University of Freiburg, Freiburg, Germany.
authors.forename: Guido
authors.initials: G
authors.lastname: Schwarzer
authors.affiliation: US Cochrane Center, John Hopkins Bloomberg School of Public Health, Baltimore, Maryland, United States of America.
authors.forename: Roberta W
authors.initials: RW
authors.lastname: Scherer
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Gerd
authors.initials: G
authors.lastname: Antes
authors.affiliation: Cochrane Switzerland, Institute of Social and Preventive Medicine (IUMSP), University Hospital Lausanne, Biopôle 2, Route de la Corniche 10, 1010 Lausanne, Switzerland.
authors.forename: Erik
authors.initials: E
authors.lastname: von Elm
authors.affiliation: German Cochrane Centre, Medical Center - University of Freiburg, Berliner Allee 29, 79110 Freiburg, Germany.
authors.forename: Joerg J
authors.initials: JJ
authors.lastname: Meerpohl
mesh_terms: D002986:Clinical Trials as Topic; D015331:Cohort Studies; D028744:Ethics Committees, Research; D006801:Humans; D011336:Probability; D017594:Publication Bias; D011642:Publications; D012042:Registries; D013997:Time Factors