Intended for healthcare professionals

Endgames Statistical Question

Internal and external validity

BMJ 2010; 340 doi: (Published 31 March 2010) Cite this as: BMJ 2010;340:c1705
  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}

    A randomised controlled trial assessed the efficacy of a psychological intervention in improving the mother-infant relationship and security of infant attachment postpartum.1 A consecutive series of 452 women in their last trimester of pregnancy who were living in poverty in a crowded South African settlement were identified. All the women were invited to take part in the study, three of whom refused. The remaining women were randomly assigned to receive the intervention (n=220) or to a control group (n=229) that received no therapeutic input.

    An analysis was conducted with data from …

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