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This article challenges the American Academy of Pediatrics recommendations for use of sensitivity and specificity in evaluating the validity of developmental screening tests. It emphasizes the clinician's need to know the prevalence of delay in the population to be screened and to estimate predictive values for that population. If one knows the prevalence, sensitivity and specificity can be useful in deriving predictive values but must meet the assumption of stability. Four common, often unrecognized, sources of instability in estimates of sensitivity and specificity are examined, namely, verification bias, small sample sizes, construct-irrelevant variance, and errors in choice and use of diagnostic procedures.