Selecting a screening cut-off point or diagnostic criterion for comparing prevalences of disease.

Comparisons of population prevalences of disease or of inadequate nutriture, across locations or over time (as in monitoring to detect a change in prevalence) are important activities in epidemiology and public health. The data collected are often counts based on a dichotomy of a continuous indicator variable, and performance of the test procedure depends on the cut-off point used. This cut-off may be chosen to optimize performance, on the assumption that the indicator has a mixed normal distribution with unknown mixing proportion corresponding to the unknown prevalence of disease. Results of James (1978, Biometrics 34, 265-275) are applied and extended to the context of monitoring. Charts are presented to facilitate determination of the optimal cut-off, and examples are given for several indicators of nutritional status.