title: Publication bias in psychology: a diagnosis based on the correlation between effect size and sample size.
abstract: BACKGROUND
  The p value obtained from a significance test provides no information about the magnitude or importance of the underlying phenomenon. Therefore, additional reporting of effect size is often recommended. Effect sizes are theoretically independent from sample size. Yet this may not hold true empirically: non-independence could indicate publication bias.
  
  
  METHODS
  We investigate whether effect size is independent from sample size in psychological research. We randomly sampled 1,000 psychological articles from all areas of psychological research. We extracted p values, effect sizes, and sample sizes of all empirical papers, and calculated the correlation between effect size and sample size, and investigated the distribution of p values.
  
  
  RESULTS
  We found a negative correlation of r = -.45 [95% CI: -.53; -.35] between effect size and sample size. In addition, we found an inordinately high number of p values just passing the boundary of significance. Additional data showed that neither implicit nor explicit power analysis could account for this pattern of findings.
  
  
  CONCLUSIONS
  The negative correlation between effect size and samples size, and the biased distribution of p values indicate pervasive publication bias in the entire field of psychology.
authors.affiliation: Department of Psychology, University of Salzburg, Salzburg, Austria; Centre for Cognitive Neuroscience, University of Salzburg, Salzburg, Austria.
authors.forename: Anton
authors.initials: A
authors.lastname: Kühberger
authors.affiliation: Österreichisches Zentrum für Begabtenförderung und Begabungsforschung, Salzburg, Austria.
authors.forename: Astrid
authors.initials: A
authors.lastname: Fritz
authors.affiliation: Department of Psychology, University of Salzburg, Salzburg, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Scherndl
mesh_terms: D015340:Epidemiologic Research Design; D006801:Humans; D001523:Mental Disorders / Q000175:diagnosis / Q000453:epidemiology; D011584:Psychology; D017594:Publication Bias; D012106:Research; D018401:Sample Size