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Endgames Statistical Question

Zelen’s design

BMJ 2012; 345 doi: (Published 14 December 2012) Cite this as: BMJ 2012;345:e8505
  1. Philip Sedgwick, reader in medical statistics and medical education1,
  2. Daniel K Sokol, medical ethicist and barrister2
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  2. 2Inner Temple, London, UK
  1. p.sedgwick{at}, daniel.sokol{at}

A randomised controlled trial incorporating Zelen’s design was used to investigate whether an intervention involving postcards reduced repetitions of deliberate self poisoning. Participants were patients aged over 16 years who presented to a toxicology service with deliberate self poisoning. The intervention consisted of eight postcards sent to the participants over 12 months, combined with standard care. Each postcard was exactly the same and invited patients to contact the toxicology service if there was anything they wished to discuss regarding their self poisoning. The control group received standard care alone. The main outcome measures were the proportion of patients having one or more repeat episodes of deliberate self poisoning and the number of repeat episodes of deliberate self poisoning per person in 12 months. In total, 772 patients were recruited, of whom 378 were randomised to intervention and 394 to control.1

The researchers reported that the postcard intervention did not significantly reduce the proportion of individual repeaters, although it significantly reduced the number of repetitions of deliberate self poisoning per participant over the 12 month study period.

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

  • a) Patients consented to being allocated to treatment at random.

  • b) All patients consented to participation in the trial.

  • c) The principle of intention to treat was used to analyse the results.


Answer c is true, whereas a and b are false.

The aim of the …

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