Intended for healthcare professionals

Endgames Statistical Question

Standardising outcome measures using z scores

BMJ 2014; 349 doi: (Published 01 October 2014) Cite this as: BMJ 2014;349:g5878
  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 if a shared care obesity management programme reduced body mass index (BMI) and related outcomes in obese children. A randomised controlled study design was used. Intervention consisted of general practice surveillance for childhood obesity, followed by obesity management across primary and tertiary care settings using a shared care model. Intervention was delivered over one year. Control consisted of “usual care.” Participants were children aged 3-10 years with a BMI above the 95th centile for their age and sex. A total of 118 children were recruited through their general practice and randomised to intervention (n=62) or control (n=56).1

The main outcome was BMI. Measurements of BMI were transformed to z scores. Secondary outcome measures included body fat percentage, waist circumference, physical activity, quality of diet, and health related quality of life. At the end of follow-up, there was no significant difference between treatment groups in BMI (adjusted mean difference −0.1 (95% confidence interval −0.7 to 0.5; P=0.7)) and BMI z score (−0.05 (−0.14 to 0.03); P=0.2). No evidence of a significant difference was found for any secondary outcome. It was concluded the shared care model of primary and tertiary care management had no effect on BMI and related outcomes in obese children.

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

  • a) The z scores permitted comparisons of BMI between children of …

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