Intended for healthcare professionals

Endgames Statistical Question

Odds ratios

BMJ 2010; 341 doi: (Published 18 August 2010) Cite this as: BMJ 2010;341:c4414
  1. Philip Sedgwick, senior lecturer in medical statistics1,
  2. Louise Marston, research statistician2
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London
  2. 2Department of Primary Care and Population Health, University College London, London
  1. Correspondence to: P Sedgwick p.sedgwick{at}

    Researchers used a case-control study to assess the risk of thrombosis associated with the use of oral contraceptives.1 Participants were premenopausal women aged less than 50 years, not pregnant, not within four weeks postpartum, and not using a hormone excreting intrauterine device or depot contraceptive.

    Women were identified as a case if they had a first objectively diagnosed episode of deep venous thrombosis or pulmonary embolism. A total of 1524 cases and 1760 otherwise healthy controls were identified. The women were then asked about use of oral contraceptives in the past year. The odds ratio of venous thrombosis for current users of oral contraceptives compared with non-users was 5.0 (95% CI 4.2 to 5.8).

    Which of the following, if any, are true?

    • a) It was possible to estimate the population at risk in this case-control study

    • b) The odds ratio is an estimate of the population relative risk

    • c) Recent …

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