title: Extent of publication bias in different categories of research cohorts: a meta-analysis of empirical studies.
abstract: BACKGROUND
  The validity of research synthesis is threatened if published studies comprise a biased selection of all studies that have been conducted. We conducted a meta-analysis to ascertain the strength and consistency of the association between study results and formal publication.
  
  
  METHODS
  The Cochrane Methodology Register Database, MEDLINE and other electronic bibliographic databases were searched (to May 2009) to identify empirical studies that tracked a cohort of studies and reported the odds of formal publication by study results. Reference lists of retrieved articles were also examined for relevant studies. Odds ratios were used to measure the association between formal publication and significant or positive results. Included studies were separated into subgroups according to starting time of follow-up, and results from individual cohort studies within the subgroups were quantitatively pooled.
  
  
  RESULTS
  We identified 12 cohort studies that followed up research from inception, four that included trials submitted to a regulatory authority, 28 that assessed the fate of studies presented as conference abstracts, and four cohort studies that followed manuscripts submitted to journals. The pooled odds ratio of publication of studies with positive results, compared to those without positive results (publication bias) was 2.78 (95% CI: 2.10 to 3.69) in cohorts that followed from inception, 5.00 (95% CI: 2.01 to 12.45) in trials submitted to regulatory authority, 1.70 (95% CI: 1.44 to 2.02) in abstract cohorts, and 1.06 (95% CI: 0.80 to 1.39) in cohorts of manuscripts.
  
  
  CONCLUSIONS
  Dissemination of research findings is likely to be a biased process. Publication bias appears to occur early, mainly before the presentation of findings at conferences or submission of manuscripts to journals.
authors: School of Medicine, Health Policy and Practice, University of East Anglia, Earlham Road, Norwich NR4 7TJ, UK. Fujian.Song@uea.ac.uk
authors: Fujian
authors: F
authors: Song
authors: Sheetal
authors: S
authors: Parekh-Bhurke
authors: Lee
authors: L
authors: Hooper
authors: Yoon K
authors: YK
authors: Loke
authors: Jon J
authors: JJ
authors: Ryder
authors: Alex J
authors: AJ
authors: Sutton
authors: Caroline B
authors: CB
authors: Hing
authors: Ian
authors: I
authors: Harvey
mesh_terms: D015331:Cohort Studies; D036262:Empirical Research; D006801:Humans; D016017:Odds Ratio; D017594:Publication Bias; D011642:Publications / Q000706:statistics & numerical data*