Endgames Statistical Question

Double dummy trials incorporating factorial designs

BMJ 2013; 346 doi: https://doi.org/10.1136/bmj.f3539 (Published 31 May 2013) Cite this as: BMJ 2013;346:f3539
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers investigated whether treatment with statins reduced blood pressure as well as cholesterol concentrations in patients being concurrently treated for mild hypertension. A randomised, placebo controlled, double blind, double dummy trial incorporating a factorial design was performed. Participants were 508 patients with mild hypertension and hypercholesterolaemia (aged 45-70 years).1

Patients were randomised to antihypertensive treatment—hydrochlorothiazide 25 mg once daily or fosinopril 20 mg once daily. The double dummy technique was used to investigate the antihypertensive treatment. In addition to the antihypertensive treatment, patients were randomised to a statin (pravastatin 40 mg once daily) or placebo.

The main outcome measures included clinic and ambulatory blood pressure measured every year. The researchers concluded that administration of a statin does not have an additional blood pressure lowering effect in hypertensive patients in whom blood pressure is effectively reduced by concomitant antihypertensive treatment.

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

  • a) To ensure that the trial was double blind, three placebos were needed …

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