Intended for healthcare professionals

Endgames Statistical Question

Confidence intervals: predicting uncertainty

BMJ 2012; 344 doi: (Published 09 May 2012) Cite this as: BMJ 2012;344:e3147
  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 evaluated the effectiveness of standardised consultations for patients with osteoarthritis of the knee.1 Standardised consultations involved three goal oriented visits, which comprised education about osteoarthritis and treatment management, as well as information on physical exercises and weight loss. A cluster randomised controlled trial was used. Control treatment was usual care. Participants were recruited from primary care. Inclusion criteria included age 45-75 years and a diagnosis by a rheumatologist of osteoarthritis of the knee according to the American College of Rheumatology clinical and radiological definition. Main outcome measures included change in body weight at four months from baseline.

In total, 336 patients were included, with 154 allocated to standardised consultation and 182 to usual care. At four months, the standardised consultation group showed greater weight loss than the usual care group (mean 1.11 kg (95% CI 0.70 to 1.52) v 0.37 kg (0.02 to 0.72); P=0.007). The authors concluded that, compared with usual care, a structured consultation programme for patients with osteoarthritis of the knee resulted in significantly greater short term weight loss.

Which one of the following statements best describes the information provided by the 95% confidence interval for mean weight loss at four months for the standardised consultation group?

  • a) 95% of sample participants in the standardised consultation group achieved a weight loss between 0.70 kg and …

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