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Endgames Statistical Question

Uncertainty in sample estimates: standard error

BMJ 2015; 350 doi: (Published 09 June 2015) Cite this as: BMJ 2015;350:h3078
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. Correspondence to: P Sedgwick p.sedgwick{at}

The efficacy of nicotine patches among pregnant smokers was investigated using a randomised placebo controlled trial. The intervention was the administration of 16 hour nicotine patches after recruitment until the time of delivery. Participants were pregnant women over 18 years who smoked at least five cigarettes a day and whose babies were between 12 and 20 weeks’ gestation. In total, 402 women were recruited from 23 maternity wards throughout France and randomly allocated to the intervention (n=203) or placebo patches (n=199).1

The outcome measures included achievement of complete abstinence until delivery and birth weight. The proportion of women who achieved complete abstinence was higher in the nicotine patch group than in the placebo group, although the difference was not significant (11 (5.5%) v 10 (5.1%); odds ratio 1.08, 95% confidence interval 0.45 to 2.60; P=0.87). The mean birth weight was higher in the nicotine patch group, although the difference was not significant (3065 g (standard error 44 g) v 3015 g (44 g); difference 50 g, −71.1 to 172.3; P=0.41). It was concluded that the nicotine patch did not increase smoking cessation rates or birth weights when compared with placebo.

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

  • a) The standard error of the mean birth weight for a treatment group provides a measure of the accuracy of the sample mean as an estimate for the population parameter.

  • b) If the sample size for the nicotine patches group increased, the size of the standard error would generally be expected to decrease.

  • c) The standard error of the …

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