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p Value fetishism and use of the Bonferroni adjustment
  1. John F Morgan
  1. Consultant Psychiatrist, Yorkshire Centre for Eating Disorders, Newsam Centre, Seacroft Hospital, Leeds LS14 6WB, UK; john.morgan@leedsmh.nhs.uk

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    The p value guides us in judgements of statistical significance, accepting or rejecting the “null hypothesis”, but can exert excessive influence over judgements of clinical significance. Karl Marx wrote of the “fetishism of commodities” in criticising the undue influence of property on capitalism’s social processes. In much the same way, evidence-based medicine has created a “fetishism of p values”. Where used to make an appraisal of the significance of difference between two groups, the p value is termed the “significance level”. The convention of relying on p<0.05 to indicate clinical significance is now deeply embedded in our critical appraisal of research papers, to such an extent that we can sometimes forget its true meaning.

    A p value less than 0.05 indicates that the differences between, say, two treatment groups will result from “chance” one time in 20, as a Type I error (signified α). This is conventionally interpreted as indicating clinical significance, useful as an aide memoire but sometimes given unwarranted influence when conceptually detached from the clinical situation in which the test has been conducted. p values must be interpreted in light of …

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