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Growth monitoring
  1. M J R HEALY
  1. 23 Coleridge Court, Milton Road
  2. Harpenden AL5 5LD, UK
  3. mjrhealy{at}compuserve.com

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    Editor,—There is a rather subtle statistical error in the Appendix to Professor Hall's paper.1 It is stated there that “if a child's height is observed to be on the 3rd centile we can be very confident that the true height lies between the 2nd and the 4th centile”. This overlooks the fact that the centile charts are derived from observed and not from true heights. If a child's height falls on the 3rd centile we can be confident that just 3% of all healthy children have observed heights which are smaller. The issue of the “true” height does not arise. A similar remark applies to velocity measurements. It is tempting to reduce the substantial measuring errors involved in observed velocities by repeating each of the two measurements and taking the averages, but different centiles would be required to interpret the results, with centile lines closer together than those on the orthodox charts.

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