Intended for healthcare professionals

Endgames Statistical Question

Bias in observational study designs: cross sectional studies

BMJ 2015; 350 doi: (Published 06 March 2015) Cite this as: BMJ 2015;350:h1286
  1. Philip Sedgwick, reader in medical statistics and medical education1
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers investigated the association between body mass index (BMI) and both sexual behaviour and adverse sexual health outcomes, as well as their importance in obese people. A telephone survey with a cross sectional study design was used. Participants were French speaking men and women aged 18-69 years who lived in France. The telephone survey was a population based one that involved a random sample of households selected from the national telephone directory. In total, 12  364 people were contacted by telephone between September 2005 and March 2006. Of those initially selected, 10 170 (82.3%) agreed to complete the questionnaire.1

An association between BMI and both sexual behaviour and adverse sexual health outcomes was reported. In particular, obese women were less likely to access contraceptive healthcare services and were more likely to have an unplanned pregnancy. It was concluded that the prevention of unintended pregnancies among obese women is a major reproductive health challenge. Healthcare professionals need to be aware of sensitivities related to weight and gender in the provision of sexual health services.

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

  • a) The use of a population based study minimised selection bias

  • b) The sample would have been prone to non-response bias

  • c) The sample was prone to volunteer bias

  • d) The results of the study were prone to ascertainment bias


Statements a, b, c, and d are all true.

The researchers performed a cross …

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