Intended for healthcare professionals

Endgames Statistical Question

Multiple significance tests: the Bonferroni correction

BMJ 2012; 344 doi: (Published 25 January 2012) Cite this as: BMJ 2012;344:e509
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers assessed the effects of hormone replacement therapy, consisting of combined oestrogen and progestogen, on health related quality of life. A randomised placebo controlled, double blind trial study design was used. Women were recruited if they were postmenopausal, had a uterus, and were aged 50-69 at randomisation. Outcome measures included health related quality of life and psychological wellbeing. The study period was one year.1

The researchers investigated the effects of combined hormone replacement therapy compared with placebo at one year, using a 0.05 (5%) critical level of significance and adjusting this with the Bonferroni correction. The researchers concluded that combined hormone replacement therapy started many years after the menopause can improve health related quality of life.

For which of the following does the Bonferroni correction reduce the probability of occurring?

  • a) Type I error

  • b) Type II error


The Bonferroni correction reduces the …

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