Dimensionality Assessment Using the Full-Information Item Bifactor Analysis for Graded Response Data

Abstract

Dimensionality assessment using the full-information item bifactor model for graded response data is provided. The model applies to data in which each item relates to a general factor and one group factor. Specifically, alternative model specification within item response theory (IRT) is shown to test a scale's factor structure. For illustrative purposes, the bifactor model and competing IRT models were fit to the data of separate cohorts of incoming college students (Cohort 1, n = 1,490; Cohort 2, n = 1,533) to test the dimensionality of an adapted version of the State Metacognitive Inventory. Overall, the bifactor analysis did not strongly support distinct group factors after accounting for the general factor. Instead, results suggested conceptualizing the scale as unidimensional, indicating that scores should be based on the total scale, not subscales. Considerations related to the use of the bifactor IRT model are discussed.