Intended for healthcare professionals

Endgames Statistical Question

What is a non-inferiority trial?

BMJ 2013; 347 doi: (Published 15 November 2013) Cite this as: BMJ 2013;347:f6853
  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 investigated the effectiveness of cognitive behavioural therapy delivered by telephone compared with the same therapy given face to face in the treatment of obsessive compulsive disorder. A randomised controlled non-inferiority trial study design was used. The intervention was 10 weekly sessions of exposure therapy and response prevention delivered by telephone or face to face. The aim of the study was to investigate whether the delivery of therapy by telephone was as effective as face to face sessions, the standard mode of delivery.1

The primary outcome measures included the self report version of the Yale Brown obsessive compulsive disorder checklist measured six months after the completion of treatment. The total score for the checklist ranges from 0 to 40, with higher scores indicating greater obsessive compulsiveness. A prespecified non-inferiority margin of 5 units on the Yale Brown checklist was proposed.

Participants were 72 patients with obsessive compulsive disorder recruited from two psychology outpatient departments. In total, 36 participants were randomised to each treatment group. Using an intention to treat analysis, the difference between treatments (face to face sessions minus telephone) in the Yale Brown checklist score at six months’ follow-up was −0.55 (95% confidence interval −4.26 to 3.15).

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

  • a) The null hypothesis states that in the population the mean Yale Brown checklist score at six months’ follow-up for standard delivery was higher than that for telephone delivery by 5 units or more

  • b) Analysis is based on the 95% confidence interval for the mean difference between treatment groups in the primary outcome

  • c) It can be concluded that telephone delivery is not inferior in effectiveness to the standard delivery of face to face sessions


Statements a, b, and c are all true.

Weekly face to face sessions were …

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