value in those settings, despite lack of strong support for the underlying theory and even in spite of threats to construct validity.

A useful analogy for understanding the issues of reliability, accuracy, and validity is the use of X-ray equipment in airport security screening. The X-ray examination is reliable if the same items are detected on repeated passes of a piece of luggage through the detection machine (test-retest reliability), if the same items are detected by different operators looking at the same image (inter-rater reliability), and if the same items are detected when the test is conducted in different ways, for example, by turning the luggage on different sides (internal consistency). The examination is accurate at detection if, in a series of tests, the X-ray image allows the examiner to correctly identify both the dangerous objects that are the targets of screening and the innocuous objects. Confidence in the validity of the test is further increased by evidence supporting the theory of X-ray screening, which includes an understanding of how the properties of various materials are registered in X-ray images. Such an understanding would increase confidence that the X-ray machine could detect not only ordinary dangerous objects, but also objects that might be concealed or altered in particular ways to avoid detection—including ways that have not yet been used in any test runs with the equipment.

For X-ray detection, as for the polygraph, reliability and validity depend both on the measuring equipment and on the capabilities and training of the operators. Validity depends on the ability of the equipment and the operators to identify target objects or conditions even when they appear in unusual ways or when efforts have been made to make them less detectable. Successful countermeasures to X-ray detection would diminish the validity of the screening. It is important to note that successful countermeasures would only decrease the test’s accuracy if they were used frequently in particular trial runs—accuracy might look quite impressive if such countermeasures had not yet been tested. This is one reason that evidence of accuracy, though necessary, is not sufficient to demonstrate test validity. X-ray screening is not presumed to have perfect validity: this is why objects deemed suspicious by X-rays are checked by direct inspection, thus reducing the number of false positive results on the X-ray examination. There is no corrective, however, for false-negative X-ray results that allow dangerous objects on an aircraft.

Measuring Accuracy

Because of the many elements that contribute to construct validity, it is difficult to represent the construct validity of a test by any single numerical indicator. This section therefore focuses on criterion validity, or accuracy, which can be measured on a single scale.



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