Statistical Question

Effect sizes

BMJ 2012; 345 doi: http://dx.doi.org/10.1136/bmj.e7370 (Published 5 November 2012)
Cite this as: BMJ 2012;345:e7370

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  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers assessed the effectiveness of an intervention designed to improve the mother-infant relationship and the security of infants in their attachment with their mother. A randomised controlled trial study design was used. Participants were pregnant women in a South African peri-urban settlement with adverse socioeconomic circumstances. The intervention, support and guidance in parenting, was delivered from late pregnancy and for six months post partum by community workers. Women in the control group received no therapeutic input.1

The main outcome measures included the quality of mother-infant interactions at six months post partum, as measured by the parent/caregiver involvement scale. The scale measured the responses of the mother to her infant’s needs and initiations—in particular, maternal sensitivity and intrusiveness. The intervention was associated with significant benefit to the mother-infant relationship. At six months post partum, compared with the controls, the intervention group were significantly more sensitive (mean difference 0.77 (SD 0.37), P<0.05, effect size d=0.24) and less intrusive (mean difference 0.68 (SD 0.36), P<0.05, effect size d=0.26) in their interactions with their infants.

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

  • a) Effect sizes are always positive in value.

  • b) Effect sizes are measured on the same scale as the outcome measure.

  • c) Effect sizes allow a direct comparison of different interventions on the same outcome.

  • d) As the difference between the intervention …

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