This discussion paper focuses on validity and reliability concerns associated with the PCK methodology. Examples of subtle validity issues from the product-testing arena are presented to illustrate the importance of validity issues, even with use of the strongest of all experimental designs, the randomized controlled clinical trial. Findings from two specific profile models are presented to illustrate the need for validating that a surrogate marker for dental caries is indeed a marker. Careful use of the two-stage PCK testing procedure is recommended, but only for intra-oral model response measures which are bona fide surrogate markers for dental caries in humans.
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