Intended for healthcare professionals

Endgames Statistical Question

Treatment allocation in trials: stratified randomisation

BMJ 2015; 350 doi: (Published 26 February 2015) Cite this as: BMJ 2015;350:h978
  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 whether the probiotic Lactobacillus reuteri reduced crying or fussing in infants with colic. A double blind placebo controlled randomised trial was performed. The intervention was oral L reuteri (1×108 colony forming units) given daily for one month. Participants were healthy term breastfed or formula fed infants, aged less than 3 months, who had infant colic. A community based sample of 167 infants was recruited and randomised in a 1:1 ratio to the intervention (n=85) or placebo (n=82). Randomisation was stratified by method of infant feeding (breast fed v formula fed) and age (≤6 weeks v >6 weeks), using a block size of two.1

The primary outcome was daily duration of crying or fussing after one month of follow-up. Secondary outcomes included the infant’s sleep duration at seven, 14, and 21 days as well as maternal mental health and family functioning at one and six months. Treatment groups were compared in outcomes using an intention to treat analysis. It was reported that the mean daily duration of crying or fussing fell steadily in both groups. At one month, the probiotic group cried or fussed for 49 minutes longer than the placebo group (95% confidence interval 8 to 90 minutes; P=0.02). There were no significant differences between treatment groups for any of the secondary outcomes. No study related adverse events were reported. The researchers concluded that L reuteri did not benefit breastfed infants or formula fed infants with colic.

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

  • a) The aim …

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