Endgames Statistical Question

What is the standard error of the mean?

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f532 (Published 29 January 2013) Cite this as: BMJ 2013;346:f532
  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

A cluster randomised double blind controlled trial investigated the effects of micronutrient supplements during pregnancy. A trial with three treatment arms was used. Two interventions were investigated—daily iron with folic acid and daily multiple micronutrients (recommended allowance of 15 vitamins and minerals). Control treatment was daily folic acid. The setting was 327 villages in two rural counties in northwest China. In total, 5828 pregnant women were recruited. Villages were randomised to treatment group, stratified by county, with a fixed ratio of treatments (1:1:1).1

Outcome measures included birth weight. Birth weight was available for analysis for 4421 live births. Mean birth weight was 3153.7 g (n=1545; 95% confidence interval 3131.5 to 3175.9, standard deviation 444.9, standard error 11.32) in the control group, 3173.9 g (n=1470; 3152.2 to 3195.6, 424.4, 11.07,) in the iron-folic acid group, and 3197.9 g (n=1406; 3175.0 to 3220.8, 438.0, 11.68) in the multiple micronutrients group. Average birth weight was significantly higher in the multiple micronutrients group than in the control (folic acid) group (difference 42.3 g; P=0.019). Although average birth weight was higher in the iron-folic acid group than in the control group, the difference was not significant (24.3 g; P=0.169).

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

  • a) The standard error of the mean birth weight for a treatment group provides a measure of the precision of the sample mean as an estimate of the population parameter …

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