Intended for healthcare professionals

Endgames Statistical Question

Before and after study designs

BMJ 2014; 349 doi: (Published 09 August 2014) Cite this as: BMJ 2014;349:g5074
  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}

Legislation to prohibit smoking in most enclosed public places was implemented in Scotland in March 2006. It has been suggested that an unintended consequence of such legislation is the displacement of adult smoking from public places into the home. Researchers investigated whether exposure to secondhand smoke in primary schoolchildren changed after the implementation of the legislation. A before and after study design was used. Participants were children in their final year of primary school, aged 10-11 years at the start of the school year. Two representative class based surveys of children were undertaken in the same schools one year apart, with one before the legislation and the other after. The surveys included different children. Of 170 schools approached, 116 agreed to take part in the study before the legislation (January 2006) and 111 of these schools also participated in the follow-up survey (January 2007). Each school selected one class to participate in each survey. A total of 2559 primary schoolchildren were surveyed in January 2006 and a further 2424 children in January 2007.1

The outcome measures included salivary cotinine concentrations and reported …

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