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Item response theory models the relationship between latent traits and responses to test items. Among other advantages, it has the ability to provide a reliable estimate of the exact score of a test-taker on the latent trait. For example, a university student's knowledge of history can be deduced from his or her score on a university test and then be compared reliably with a high school student's knowledge deduced from a less difficult test. Scores derived by classical test theory do not have this characteristic, and assessment of actual ability (rather than ability relative to other test-takers) must be assessed by comparing scores to those of a norm group randomly selected from the population. In fact, all measures derived from classical test theory are dependent on the sample tested, while those derived from item response theory are not. Predictive or concurrent validity cannot exceed the square of the correlation between two versions of the same measure.
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