Adaptive Estimation When the Unidimensionality Assumption of IRT is Violated

This study examined some effects of using a unidi mensional IRT model when the assumption of unidi mensionality was violated. Adaptive and nonadaptive tests were formed from two-dimensional item sets. The tests were administered to simulated examinee populations with different correlations of the two un derlying abilities. Scores from the adaptive tests tended to be related to one or the other ability rather than to a composite. Similar but less disparate results were obtained with IRT scoring of nonadaptive tests, whereas the conventional standardized number-correct score was equally related to both abilities. Differences in item selection from the adaptive administration and in item parameter estimation were also examined and related to differences in ability estimation. Index terms: ability estimation, adaptive testing, item pa rameter estimation, item response theory, multidimen sionality.