As shown in Chapter 2, a similar tradeoff of specificity for sensitivity can be obtained with a single test based on a continuous measurement by changing the cutoff or threshold used for classification on the basis of the test result and thus moving to the right on the receiver operating characteristic (ROC) curve for that test. The virtue of the parallel combination is that it brings more information to bear on the problem. Hence, if one begins with component tests of fixed cutoff points and generates the parallel combination test from them, the result will have a greater sensitivity and lower specificity than any of the component tests using the same cutoff point. In general, sensitivity is the test characteristic that most strongly drives negative predictive value, which in turn governs the ability to rule out a diagnosis. Hence, negative parallel tests are often used in medical care for the explicit purpose of excluding a disease diagnosis.
If the component tests each have some discriminating power, the parallel test will often also have a greater sensitivity than any component test calibrated to the specificity achieved by the combination. The gain in accuracy, however, is limited by the degree to which each new test in the parallel combination is correlated with the feature one is trying to detect. Any dependence between tests would reduce the amount of new information available, and consequently, diminish the potential gain. With many tests, it is unlikely that the best discriminating function will be obtained by requiring that a person is classified negative only if all tests are negative—better decision rules will come from the various classification methods listed above.
The independent parallel testing argument suggests that polygraph testing might be useful in the security screening context even if it were not sufficiently valid by itself to be useful. A negative polygraph examination combined with other negative data might increase the certainty of a decision to grant or continue access to sensitive information. The degree to which the polygraph improved the decision-making process in such a context, however, would depend on whether polygraph test results can appropriately be treated as statistically independent of other screening modalities, as well as on the discriminating power of the polygraph. The false positive rate of the parallel combination will exceed that of any component, so the polygraph cutoff in a parallel investigation might have to be set accommodate this (that is, to increase the range of scores considered as indicating truthfulness) with a corresponding sacrifice in sensitivity.
In independent serial testing a sequence of tests is specified, with each test used only if its predecessors in the sequence have all been positive. Serial tests are the general rule in medical practice, especially if one