In Reply.—Dr Piper and colleagues raise several issues that illustrate the difficulties of applying clinical epidemiologic methods to the field of developmental disabilities. Most tests in the clinical laboratory provide either a positive or negative result. Very few clinically useful tests give an indeterminate result. In the field of developmental disabilities, we frequently encounter children with developmental delay who do not fit more specific disease categories. In Harris' article it is unclear whether the children with developmental delay should be included in the "diseased" or "nondiseased" group. For the sake of completeness, presenting test results for these children would allow the clinician to decide in which group to include them. However, if we use Piper and colleagues' suggestion to place these children in the "nondiseased" group, this would have no effect on the sensitivity calculations shown in the article.
As Piper and colleagues indicate, the predictive value of a