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Endgames Statistical Question

Randomised controlled trials: balance in baseline characteristics

BMJ 2014; 349 doi: (Published 19 September 2014) Cite this as: BMJ 2014;349:g5721
  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 investigated the effectiveness of helmet therapy for positional skull deformation in infants aged 5-6 months. A randomised controlled trial study design was used. Helmet therapy was delivered for six months. Control treatment was no treatment, which consisted of the natural course of skull deformation. Participants were 84 infants aged 5-6 months with moderate to severe skull deformation. Infants were assigned to helmet therapy (n=42) or the natural course of the condition (n=42) using random allocation with a block size of eight.1

The main outcomes were anthropometric measurements of the change in skull shape from baseline to 24 months of age, including the oblique diameter difference index and cranioproportional index. Statistical hypothesis testing was used to compare the treatment groups in baseline characteristics, including sex distribution, age, birth rank, health problems, and ethnicity, plus education level of the parents. The researchers reported that the treatment groups were comparable in baseline characteristics.

There were no significant differences between the treatment groups in the main outcome measures. Furthermore, helmet therapy was associated with a high prevalence of side effects. Because helmet therapy also involved high costs, the researchers discouraged the use of this treatment in healthy infants with moderate to severe skull deformation.

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

  • a) The purpose of randomisation was to achieve treatment groups with similar baseline characteristics

  • b) Block randomisation was used to achieve similar numbers of participants in the treatment groups

  • c) …

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