Intended for healthcare professionals

Endgames Statistical Question

Bias in randomised controlled trials: comparison of crossover group and parallel group designs

BMJ 2015; 351 doi: (Published 07 August 2015) Cite this as: BMJ 2015;351:h4283
  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 and side effects of the synthetic cannabinoid nabilone were compared with those of the weak opioid dihydrocodeine in the treatment of chronic neuropathic pain. A randomised, double blind, crossover trial study design was used. Participants received a maximum daily dose of 2 mg nabilone or 240 mg dihydrocodeine. The trial lasted for 14 weeks and comprised two treatment periods, each of six weeks’ duration, separated by a two week washout period. Participants were patients with chronic neuropathic pain aged 23-84 years, who were recruited using convenience sampling from outpatient units at three hospitals in the United Kingdom. The sample comprised 96 patients, who were randomised to nabilone (n=48) and dihydrocodeine (n=48) in the first treatment period.1

The main outcome was pain as measured on a visual analogue scale over the final two weeks of each treatment period. Side effects were measured by a questionnaire. The researchers reported that dihydrocodeine provided better pain relief than nabilone and had slightly fewer side effects, although no major adverse events occurred for either drug.

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

  • a) The crossover design is referred to as a “within subjects” study design

  • b) The sample was prone to selection bias

  • c) The crossover trial was prone to a carryover effect

  • d) The crossover trial would be expected to take longer than the equivalent parallel groups design

  • e) The crossover trial design would be expected to promote internal validity compared with the equivalent parallel groups design


Statements a, b, c, d, and e are all true.

The aim of the trial was to compare the efficacy and side effects of nabilone with dihydrocodeine in the treatment of chronic neuropathic pain. A randomised double blind crossover trial was conducted. Participants received both treatments in succession, …

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