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The a parameter, or the discrimination parameter, is one of the key item parameters in many item response theory (IRT) models. This entry discusses how the a parameter is defined and interpreted. For this discussion, properties of the a parameter are introduced in the unidimensional IRT framework in which items in a test measure one and only one construct, with a focus on dichotomous item responses. Realistically, items in a test are considered to be unidimensional as long as a single construct accounts for a substantial portion of the total score variance.

Unidimensional IRT framework is the focus of this entry because it is the foundation of its multidimensional counterpart, and basic principles in unidimensional IRT framework can be straightforwardly interpreted in the multidimensional context. In ...

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