Statistical Question

Parametric statistical tests for independent groups: numerical data

BMJ 2012; 345 doi: http://dx.doi.org/10.1136/bmj.e8145 (Published 30 November 2012)
Cite this as: BMJ 2012;345:e8145

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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 investigated whether a school based educational programme aimed at reducing consumption of carbonated drinks prevented excessive weight gain in children. A cluster randomised controlled trial study design was used. The intervention, which was delivered over one school year, included focused education promoting a healthy diet together with discouragement of carbonated drink consumption. The control group received no intervention. Children were followed for three years from baseline.1

The main outcome measures included body mass index (BMI) converted to age and sex specific z scores. A total of 644 children aged between 7 and 11 years from six schools were recruited. Measurements were obtained from 434 children three years after baseline. Distributional assumptions of normality in the BMI z scores were verified. At follow-up the age and sex specific BMI z scores had increased in the control group by a mean of 0.1 (SD 0.53) but decreased in the intervention group by 0.01 (SD 0.58). The mean difference between treatment groups was not significant (0.1 (95% confidence interval 0 to 0.21); P=0.06).

Which one of the following statistical tests was most likely used to compare treatment groups in the mean difference in the mean change in BMI z score over three years from baseline?

  • a) Paired t test

  • b) Student’s t test

  • c) Wilcoxon rank sum test

  • d) Wilcoxon signed ranks test

Answers

Student’s t test (answer b) would most likely have been used to compare the …

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