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Endgames Statistical Question

Relative risks and statistical significance

BMJ 2010; 341 doi: (Published 11 August 2010) Cite this as: BMJ 2010;341:c4265

This article has a correction. Please see:

  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}

    Previous questions described the interpretation of a relative risk and associated 95% confidence interval.1 2 One of the examples used was a prospective cohort study, which assessed the effects of β lactam antibiotics prescribed in the community for acute respiratory tract infection on the prevalence of antibiotic resistant bacteria in children. Antibiotic resistance was assessed by the presence of the ICEHin1056 resistance element in up to four isolates of Haemophilus species recovered from throat swabs at recruitment and follow-up.3

    At two weeks, Haemophilus possessing homologues of ICEHin1056 was isolated from 67% of children prescribed antibiotics compared with 36% of those not prescribed antibiotics (relative risk 1.9, 95% confidence interval 1.2 to 2.9). Tests of statistical significance were two sided.

    Which of the following, if any, are true?

    • a) Null hypothesis: in the population of …

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