Intended for healthcare professionals

Endgames Statistical Question

Explanatory trials versus pragmatic trials

BMJ 2014; 349 doi: (Published 13 November 2014) Cite this as: BMJ 2014;349:g6694
  1. Philip Sedgwick, reader in medical statistics and medical education1
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}

Researchers assessed the effectiveness of weekly delivery of low dose high frequency therapeutic ultrasound in conjunction with standard care for hard to heal venous leg ulcers. A multicentre pragmatic two arm randomised controlled trial study design was used. Ultrasound was delivered during weekly dressing changes, and the duration depended on the size of the ulcer. The control treatment was standard care alone, comprising low adherent dressings and four layer bandaging that was high compression, reduced compression, or no compression, depending on the patient’s tolerance. Treatment was delivered for a maximum of 12 weeks.1

Participants were 337 patients with at least one venous leg ulcer of more than six months’ duration or greater than 5 cm2 in area, and an ankle brachial pressure index of 0.8 or more. The primary outcome was time to healing of the largest eligible leg ulcer. Secondary outcomes included health related quality of life and adverse events. Participants were followed for a maximum of 12 months.

The researchers reported that low dose high frequency ultrasound given weekly for 12 weeks during dressing changes in addition to standard care did not significantly increase ulcer healing rates, affect quality of life, or reduce ulcer recurrence.

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

  • a) As a pragmatic trial, it would have been undertaken in a routine clinical and healthcare setting

  • b) The participants would have been monitored closely to ensure adherence to their allocated treatment regimen

  • c) The trial would be expected to have high external validity

  • d) The trial would be expected to have high internal validity


Statements a and c are true, whereas b and d are false.

Clinical trials are described as explanatory or pragmatic. Explanatory trials generally measure efficacy—that is, the benefit of …

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