Intended for healthcare professionals

Endgames Statistical Question

Standardisation of outcome measures (z scores)

BMJ 2012; 345 doi: (Published 14 September 2012) Cite this as: BMJ 2012;345:e6178
  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}

Researchers investigated the effects of weekly vitamin D supplements on the mortality, morbidity, and growth of low birthweight term infants up to 6 months of age in India. A randomised placebo controlled trial was used. In total, 2079 low birthweight infants born at term (>37 weeks’ gestation) were randomised to intervention or control. Intervention consisted of weekly vitamin D supplements for six months at a dose of one recommended nutrient intake per day (35 μg/week) dissolved in a small amount of expressed breast milk (or other milk for non-breastfed infants).1

Primary outcomes were admission to hospital or death during the first six months of life. Secondary outcomes included weight, length, mid-upper arm circumference, and head circumference at six months. Each anthropometric measure was transformed to z scores for age and sex using World Health Organization growth reference charts. The researchers reported that a weekly dose of vitamin D in young …

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