Authors' reply

BMJ 1995; 311 doi: https://doi.org/10.1136/bmj.311.7009.875 (Published 30 September 1995) Cite this as: BMJ 1995;311:875
  1. T D Spector,
  2. R W Keen,
  3. N K Arden,
  4. T V Nguyen,
  5. N A Morrison,
  6. P N Sambrook,
  7. P J Kelly,
  8. J A Eisman
  1. Consultant rheumatologist Research fellow Research fellow Rheumatology Department, St Thomas's Hospital, London SE1 7EH
  2. Statistician Senior scientist Associate professor Senior lecturer Professor Bone and Mineral Research Division, Garvan Institute of Medical Research, Sydney, Australia

    EDITOR,--Munro Peacock and colleagues' discussion of the three published sets of data on twins apparently ignores the increasing number of studies that have confirmed the association between vitamin D receptor genotype and bone mineral density.1 2 3

    Although the intraclass correlation in dizygotic twin pairs concordant for vitamin D receptor genotype was consistently higher than that observed in the dizygotic twin pairs that were discordant for the genotype, none of these differences reached significance. Intraclass correlation, however, is a measure of similarity, not of magnitude or direction of effect. Our conclusion of association (not linkage) between vitamin D receptor genotypes and bone mineral density was based not on the analysis of differences between intraclass correlations but on the analysis of co-variance of all …

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