Intended for healthcare professionals

Endgames Statistical Question

Standard deviation versus standard error

BMJ 2011; 343 doi: (Published 13 December 2011) Cite this as: BMJ 2011;343:d8010
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers investigated the effectiveness of a weight loss programme in men with moderate to severe obstructive sleep apnoea. Men were eligible to join the programme if aged 30-65 years, had a body mass index between 30 and 40, had moderate to severe obstructive sleep apnoea (as measured by an apnoea-hypopnoea index ≥15 events/hour), and were being treated with continuous positive airway pressure. The weight loss programme lasted one year and consisted of a very low energy diet for nine weeks followed by a weight loss maintenance programme. A total of 63 men were recruited at an outpatient obesity clinic in a university hospital in Stockholm, Sweden.1

Outcome measures included the change in body weight after participation in the weight loss programme. At baseline the sample had a mean weight of 113.1 kg (SD=14.2 kg). The researchers reported that the weight loss programme resulted in a significant decrease in weight. The mean change in body weight at one year from baseline was a reduction of 12.1 kg (95% confidence interval 9.8 to 14.3) (SD=9 kg; SEM=1.13 kg).

Which of the following, if any, are true?

  • a) The standard deviation of body weight at baseline provides a measure of the spread of observations of weight in the sample before participants began the weight loss programme.

  • b) At baseline, approximately 95% of sample members had a body weight that was within two standard …

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