This paper proposes a simple approach to the parametric derivation of age-related reference ranges which avoids the creation of arbitrary age groups, copes easily with a non-linear relation between variability and age, and is computationally simple. After the mean is modelled as a function of age, the age-specific standard deviation is estimated by regressing the absolute residuals on age. The method assumes that the data are, or can be transformed to be, normal at each age, and uses the properties of the half normal distribution. An example is given using 450 measurements of fetal foot length.
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