Endgames Statistical Question

T scores and z scores

BMJ 2010; 341 doi: http://dx.doi.org/10.1136/bmj.c7362 (Published 30 December 2010) Cite this as: BMJ 2010;341:c7362
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Section of Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

A 79 year old white woman had her bone mineral density (BMD) measured to assess for osteoporosis and fracture risk. The table shows the results of dual energy x ray absorptiometry (DXA).

View this table:

Results of dual energy x ray absorptiometry on the right hip of a 79 year old patient

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

  • a) The z scores enable BMD measurements to be compared between men and women of different ages and ethnic groups

  • b) The z scores are normally distributed

  • c) This woman’s BMD measurements were below the mean for her age and ethnicity

  • d) The T scores compare the woman’s BMD measurements with those of white women at an age when bone density is at its peak

Answers

Answers a, b, and d are true, whereas c is false.

Measurement of BMD in the hip and spine is the gold standard for the diagnosis of osteoporosis and assessment of fracture risk. BMD is the ratio of bone mineral content to bone area (expressed in g/cm²). Measurements for three regions of the woman’s right hip are given—the femoral neck, tronchanteric region, and intertrochanteric region. A total BMD measurement was obtained by scanning the entire hip …

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