Are vocabulary tests measurement invariant between age groups? An item response analysis of three popular tests

Relatively high vocabulary scores of older adults are generally interpreted as evidence that older adults possess more of a common ability than younger adults. Yet, this interpretation rests on empirical assumptions about the uniformity of item-response functions between groups. In this article, we test item response models of differential responding against datasets containing younger-, middle-aged-, and older-adult responses to three popular vocabulary tests (the Shipley, Ekstrom, and WAIS-R) to determine whether members of different age groups who achieve the same scores have the same probability of responding in the same categories (e.g., correct vs. incorrect) under the same conditions. Contrary to the null hypothesis of measurement invariance, datasets for all three tests exhibit substantial differential responding. Members of different age groups who achieve the same overall scores exhibit differing response probabilities in relation to the same items (differential item functioning) and appear to approach the tests in qualitatively different ways that generalize across items. Specifically, younger adults are more likely than older adults to leave items unanswered for partial credit on the Ekstrom, and to produce 2-point definitions on the WAIS-R. Yet, older adults score higher than younger adults, consistent with most reports of vocabulary outcomes in the cognitive aging literature. In light of these findings, the most generalizable conclusion to be drawn from the cognitive aging literature on vocabulary tests is simply that older adults tend to score higher than younger adults, and not that older adults possess more of a common ability.