Intended for healthcare professionals

Endgames Statistical Question

Skewed distributions II

BMJ 2010; 341 doi: (Published 17 November 2010) Cite this as: BMJ 2010;341:c6448
  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}

Researchers investigated the effects of long term treatment with metformin on the risk of vitamin B12 deficiency in patients with type 2 diabetes receiving insulin treatment.1 A multicentre randomised placebo controlled trial was used. A total of 390 patients were randomly assigned to receive 850 mg metformin or 850 mg placebo three times a day.

Primary outcome measures included percentage change in the serum concentration of homocysteine from baseline at 4, 17, 30, 43, and 52 month follow-up. At baseline, the metformin group (n=196) had a mean homocysteine concentration of 14.4 μmol/l (standard deviation 9.7 μmol/l), compared with a mean concentration of 14.6 μmol/l (standard deviation 10.3 μmol/l) for the placebo group (n=194).

Which of the following statements, if any, are true for the metformin group at baseline?

  • a) Approximately 68% or more of the homocysteine measurements were no further than one standard deviation away from the sample mean …

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