Endgames Statistical Question

Numbers needed to treat and harm

BMJ 2010; 341 doi: http://dx.doi.org/10.1136/bmj.c5731 (Published 20 October 2010) Cite this as: BMJ 2010;341:c5731
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Section of Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

The effects of statins in people aged between 30 and 84 years were investigated using a prospective cohort study.1 Data were taken from the QResearch database, a computerised database of anonymised longitudinal medical records collected prospectively in primary care. A cohort of patients registered with general practices between 1 January 2002 and 30 June 2008 was used. New users of statins were compared with non-users.

The intended effect of statins was a reduction in the occurrence of cardiovascular disease. The number needed to treat for cardiovascular disease over five years was 37 (95% confidence interval 27 to 64) for women and 33 (95% CI 24 to 57) for men. Acute renal failure was a side effect of statins: the number needed to harm was 434 (95% CI 284 to 783) for women and 593 (95% CI 388 to 1070) for men.

Which of the following statements, if any, are true?

  • a) The number needed to treat is the number of individuals who need to be treated with statins over five years for one to experience cardiovascular disease

  • b) The number needed to harm is the number of individuals that would need to be treated with statins over five years for one more person to experience acute renal failure than would …

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