Statistical Question

Intraclass correlation coefficient

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f1816 (Published 22 March 2013)
Cite this as: BMJ 2013;346:f1816

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  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 the association between bone mineral density measurements in twins, with monozygotic (identical) and dizygotic (non-identical) twins studied separately. The comparison was part of a study that investigated the association between vitamin D receptor genotype and bone mineral density in postmenopausal twins. A cross sectional study design was used, with participants recruited from a national register of twins in Britain. Participants were 87 monozygotic and 95 dizygotic pairs of twins aged 50-69 years, postmenopausal, and free of diseases affecting bone.1

The main outcome measures included lumbar spine bone mineral density as measured by dual energy x ray absorptiometry. The intraclass correlation coefficient for lumbar spine bone mineral density between monozygotic twins was higher than for dizygotic ones (0.74 v 0.36). It was concluded that in British postmenopausal women’s lumbar spine bone mineral density was genetically linked.

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

  • a) The intraclass correlation coefficient provided a measure of agreement between measurements of lumbar spine bone mineral density for sets of twins

  • b) The intraclass correlation coefficient can take a value from −1 through 0 to +1

  • c) Agreement between lumbar spine bone mineral density measurements was greater for monozygotic twins than for dizygotic ones

Answers

Statements a and c are …

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