Power Rate Prediction Instrumentation

Such considerations have led to development of the concept of predictive power, which is more directly useful in evaluating the results from a screening instrument. Positive predictive power is the probability that a child identified by the instrument as having the disorder or symptom actually does have it, and negative predictive power is the probability that a child identified as not having the disorder or symptom actually does not have it. In some ways, predictive power is the converse of specificity and sensitivity, but it usually also depends on the base rate. If sensitivity is 100% then the negative predictive power would be 100%. If specificity is 100%, then positive predictive power is 100%. But when sensitivity and specificity are less than 100% (the usual situation), the base rate enters into the calculation. With 90% sensitivity, 90% specificity, and a 5% base rate, there are 4.5% true positives, 85.5%... 

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