Intended for healthcare professionals

Endgames Statistical Question

Confounding in case-control studies

BMJ 2010; 341 doi: (Published 22 September 2010) Cite this as: BMJ 2010;341:c5136
  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}

    Last week’s question described a population based case-control study that investigated whether infants’ sleeping environment was a risk factor for sudden infant death syndrome.1 2 Four controls matched for age, locality, and time of sleep were selected for each case. Infants’ usual sleeping environment in relation to their parents was classified as usual room sleeper (shared room but not the same bed), usual solitary sleeper (slept in room separate from parents), or usual bed sharer (shared bed with parents for more than two nights per week).

    The researchers also investigated whether parents’ social class was associated with sudden infant death syndrome. The results are presented in the table, which shows the odds ratios of sudden unexpected death for the categories of the infants’ usual sleeping environment for all cases and controls, stratified by socioeconomic class.

    View this table:

    Infants’ sleeping environment in relation to parents’ usual night time practice, by socioeconomic group

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

    • a) Compared with usual room sharers, usual bed sharers were statistically significantly more likely to experience sudden infant death …

    View Full Text

    Log in

    Log in through your institution


    * For online subscription