title: Excess significance bias in the literature on brain volume abnormalities.
abstract: BACKGROUND
  Many studies report volume abnormalities in diverse brain structures in patients with various mental health conditions.
  
  
  OBJECTIVE
  To evaluate whether there is evidence for an excess number of statistically significant results in studies of brain volume abnormalities that suggest the presence of bias in the literature.
  
  
  METHODS
  PubMed (articles published from January 2006 to December 2009).
  
  
  METHODS
  Recent meta-analyses of brain volume abnormalities in participants with various mental health conditions vs control participants with 6 or more data sets included, excluding voxel-based morphometry.
  
  
  METHODS
  Standardized effect sizes were extracted in each data set, and it was noted whether the results were "positive" (P < .05) or not. For each data set in each meta-analysis, I estimated the power to detect at α = .05 an effect equal to the summary effect of the respective meta-analysis. The sum of the power estimates gives the number of expected positive data sets. The expected number of positive data sets can then be compared against the observed number.
  
  
  RESULTS
  From 8 articles, 41 meta-analyses with 461 data sets were evaluated (median, 10 data sets per meta-analysis) pertaining to 7 conditions. Twenty-one of the 41 meta-analyses had found statistically significant associations, and 142 of 461 (31%) data sets had positive results. Even if the summary effect sizes of the meta-analyses were unbiased, the expected number of positive results would have been only 78.5 compared with the observed number of 142 (P < .001).
  
  
  CONCLUSIONS
  There are too many studies with statistically significant results in the literature on brain volume abnormalities. This pattern suggests strong biases in the literature, with selective outcome reporting and selective analyses reporting being possible explanations.
authors.affiliation: Clinical and Molecular Epidemiology Unit, Department of Hygiene and Epidemiology, University of Ioannina School of Medicine, and Biomedical Research Institute, Foundation for Research and Technology–Hellas, Ioannina, Greece. jioannid@stanford.edu
authors.forename: John P A
authors.initials: JP
authors.lastname: Ioannidis
mesh_terms: D001921:Brain / Q000473:pathology*; D006801:Humans; D001523:Mental Disorders / Q000473:pathology; D015201:Meta-Analysis as Topic; D017594:Publication Bias / Q000706:statistics & numerical data*