Intended for healthcare professionals

Endgames Statistical Question

Prospective cohort studies: advantages and disadvantages

BMJ 2013; 347 doi: (Published 08 November 2013) Cite this as: BMJ 2013;347:f6726
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers investigated the association between opium use and subsequent risk of death. A prospective cohort study design was used. Participants were 50 045 people in north-eastern Iran aged 40-75 years at baseline. Recruitment took place between January 2004 and June 2008, and participants were followed until May 2011. The median length of follow-up was 4.7 years per participant. The main outcomes were death from all causes, plus all major subcategories.1

Information about opium use was collected at baseline. Participants were asked their age when they started using opium and subsequent length of use, typical amount used, frequency of use, and routes of administration. Information about exposure to a wide variety of other risk factors, including tobacco smoking and alcohol consumption, was also collected at baseline. During follow-up, participants were contacted annually by telephone with detailed questions about their health status and any hospital admissions or outpatient procedures. Opium use and exposure to other risk factors were not systematically updated.

The study concluded that opium users have an increased risk of death from multiple causes compared with non-users. Increased risks were also seen in people who had used low amounts of opium for a long period, plus those who had no major illness before use.

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

  • a) Recall bias was minimised

  • b) It was possible to estimate the population at risk

  • c) It can be inferred that opium use causes an increased risk of death

  • d) The results may be biased if a substantial number of cohort members were lost to follow-up


Statement a, b, and d …

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