title: The Bayesian interpretation of a P-value depends only weakly on statistical power in realistic situations.
abstract: OBJECTIVE
  It is often repeated that a low P-value provides more persuasive evidence for a genuine effect if the power of the test is high. However, this is based on an argument which ignores the precise P-value in favor of simply observing whether P is less than some cut-off, and which oversimplifies the possible effect sizes. In a non-Bayesian framework, there are good reasons to think that power does not affect the evidence of a given P-value. Here I illustrate the relationship between pre-study power and the Bayesian interpretation of a P-value in realistic situations.
  
  
  METHODS
  A Bayesian calculation, using a conventional prior distribution for the effect size and a normal approximation to the sampling distribution of the sample estimate, where the datum is the precise P-value.
  
  
  RESULTS
  Over the range of pre-study powers typical in published research, the Bayesian interpretation of a given P-value varies little with power.
  
  
  CONCLUSIONS
  A Bayesian analysis with reasonable assumptions produces results remarkably in line with a more simple, non-Bayesian intuition-that the evidence against the null hypothesis provided by a precise P-value should not depend on power.
authors.affiliation: National Heart & Lung Institute, Imperial College London, UK. richard.hooper2@imperial.ac.uk
authors.forename: Richard
authors.initials: R
authors.lastname: Hooper
mesh_terms: D001499:Bayes Theorem; D003627:Data Interpretation, Statistical; D003955:Diagnostic Tests, Routine; D006801:Humans; D012107:Research Design