Intended for healthcare professionals

Endgames Statistical Quiz

Observational study design II

BMJ 2011; 342 doi: (Published 31 March 2011) Cite this as: BMJ 2011;342:d1903
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London
  1. p.sedgwick{at}

Researchers investigated whether the use of oral or transdermal hormone replacement therapy was a risk factor for stroke. Data were taken from the General Practice Research Database, a computerised database of anonymised longitudinal medical records collected prospectively in primary care. A cohort of women was identified, aged 50-79 years with records between 1 January 1987 and 31 October 2006, and without a diagnosis of stroke on the date of registration with their general practice. A woman was identified as a case if she experienced a stroke during follow-up, and up to four controls were randomly selected from the cohort. Controls were matched to cases on age (within one year) at the date of diagnosis of the stroke, general practice where registered, and the year of joining the practice. The risk of stroke was increased with oral hormone replacement therapy of any dose but only with transdermal patches containing high doses of oestrogen.1

Which of the following best describes …

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