title: The distribution of probability values in medical abstracts: an observational study.
abstract: BACKGROUND
  A relatively high incidence of p values immediately below 0.05 (such as 0.047 or 0.04) compared to p values immediately above 0.05 (such as 0.051 or 0.06) has been noticed anecdotally in published medical abstracts. If p values immediately below 0.05 are over-represented, such a distribution may reflect the true underlying distribution of p values or may be due to error (a false distribution). If due to error, a consistent over-representation of p values immediately below 0.05 would be a systematic error due either to publication bias or (overt or inadvertent) bias within studies.
  
  
  METHODS
  We searched the Medline 2012 database to identify abstracts containing a p value. Two thousand abstracts out of 80,649 abstracts were randomly selected. Two independent researchers extracted all p values. The p values were plotted and compared to a predicted curve. Chi square test was used to test assumptions and significance was set at 0.05.
  
  
  RESULTS
  2798 p value ranges and 3236 exact p values were reported. 4973 of these (82%) were significant (<0.05). There was an over-representation of p values immediately below 0.05 (between 0.01 and 0.049) compared to those immediately above 0.05 (between 0.05 and 0.1) (p = 0.001).
  
  
  CONCLUSIONS
  The distribution of p values in reported medical abstracts provides evidence for systematic error in the reporting of p values. This may be due to publication bias, methodological errors (underpowering, selective reporting and selective analyses) or fraud.
authors.affiliation: Department of Orthopaedics, The Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia. basginsel@gmail.com.
authors.forename: Bastiaan
authors.initials: B
authors.lastname: Ginsel
authors.affiliation: Department of Orthopaedics, The Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia. aggarwal.abhinav@gmail.com.
authors.forename: Abhinav
authors.initials: A
authors.lastname: Aggarwal
authors.affiliation: Department of Biostatistics, The Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia. wei.xuan@sswahs.nsw.gov.au.
authors.forename: Wei
authors.initials: W
authors.lastname: Xuan
authors.affiliation: Department of Orthopaedics, The Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia. iaharris1@gmail.com.
authors.forename: Ian
authors.initials: I
authors.lastname: Harris
mesh_terms: D000043:Abstracting and Indexing / Q000706:statistics & numerical data*; D016009:Chi-Square Distribution; D006801:Humans; D016239:MEDLINE / Q000706:statistics & numerical data*; D066295:Open Access Publishing / Q000706:statistics & numerical data; D011336:Probability; D017594:Publication Bias / Q000706:statistics & numerical data*