Sir.—The report by Charrow and Hedges1 in the February 1990 issue of AJDC contains a conceptual error. The method for calculating posterior probability (PP) of noncarrier and carrier status at a given assay value, as used in the report, actually gives the PP for all values above or below the value, depending on which status one is assessing.
The PP (sometimes called the likelihood ratio) at a given value equals the ratio of the heights of the normal curves at that value.2 These heights, given the means and SDs for both affected and unaffected populations, can be calculated from this formula:
in which SD is the standard deviation, xi is the observed value, and x is the mean for the given population.
Charrow and Hedges did not give sufficient data to calculate the PP for the example levels of hexaminidase A used in their report. By crude
JOHNSON AM. Abnormal Test Results?. Am J Dis Child. 1990;144(10):1069. doi:10.1001/archpedi.1990.02150340011003