Intended for healthcare professionals

Endgames Statistical Question

Prevalence and incidence

BMJ 2010; 341 doi: (Published 01 September 2010) Cite this as: BMJ 2010;341:c4709
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London
  1. p.sedgwick{at}

    A randomised controlled trial investigated whether screening and treating women for chlamydial infection reduced the subsequent occurrence of pelvic inflammatory disease.1 Between 2004 and 2006, vaginal swabs were provided by 2529 sexually active female students. The samples were randomly allocated to immediate testing (screening group), or storage and deferred screening after a year (control group). After screening, treatment for chlamydial infection was offered to women where necessary. All women were able to undertake independent testing for chlamydia during the follow-up period of one year.

    At baseline, 5.4% of the women in the screened group tested positive for chlamydia compared to 5.9% of the controls. Over the following twelve months, 1.3% of the screened women developed pelvic inflammatory disease compared with 1.9% of the controls.

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

    • a) The percentage of women with chlamydia at baseline estimated the population prevalence

    • b) Women who developed pelvic inflammatory disease during follow-up are known as incident cases

    • c) …

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