Intended for healthcare professionals

Endgames Statistical Question

What is a crossover trial?

BMJ 2014; 348 doi: (Published 09 May 2014) Cite this as: BMJ 2014;348:g3191
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers evaluated the effectiveness of the cannabinoid dronabinol on central neuropathic pain in patients with multiple sclerosis.[1] The effectiveness of cannabinoids in relieving pain caused by central lesions in multiple sclerosis had not been investigated previously. A randomised double blind placebo controlled crossover trial study design was used. The intervention was orally administered dronabinol at a maximum dose of 10 mg daily or corresponding placebo. Each treatment period was for three weeks, separated by a three week washout period. All analgesic drugs, except for paracetamol, were discontinued at least one week before the start of the trial.

The primary outcome was spontaneous pain intensity in the last week of each treatment period measured using a numerical rating scale. The researchers reported that, when compared with placebo, dronabinol had a significant analgesic effect on central pain in patients with multiple sclerosis. Although the effects were modest they were clinically relevant. The number of patients with adverse events was higher during active treatment, especially in the first week of treatment. The functional ability of the patients with regard to their multiple sclerosis did not change.

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

  • a) All participants received both treatments in the same sequence order

  • b) Each participant acted as his or her own control

  • c) The purpose of the washout period was to allow the effects of the first treatment to dissipate before starting the second treatment

  • d) The crossover study design required more participants than if a “between subjects” study design had been …

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