Endgames Statistical Question

z scores

BMJ 2010; 341 doi: http://dx.doi.org/10.1136/bmj.c6746 (Published 01 December 2010) Cite this as: BMJ 2010;341:c6746

This article has a correction. Please see:

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

A cluster randomised controlled trial assessed the long term effects of an obesity prevention programme in schools.1 A total of 511 children aged between 7 and 11 years from six schools were recruited and randomly allocated to the intervention or control. The intervention comprised focused education promoting a healthy diet plus discouragement of consumption of carbonated drinks, and was delivered over one school year. The control group received no intervention. Both groups were followed up three years after randomisation.

The primary outcome was change in body mass index (BMI) at follow-up. BMI was transformed to z scores using previously published growth reference curves that adjusted for age and sex. At three year follow-up, the BMI z scores had increased in the control group by 0.10 (SD 0.53) but decreased in the intervention group by −0.01 (SD 0.58), with a mean difference of 0.10 (95% confidence interval −0.00 to 0.21; P=0.06).

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

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