Bifactor Item Response Theory (IRT) models are presented as a plausible structure for psychological measures with a primary scale and two or more subscales. A bifactor graded response model, appropriate for polytomous categorical data, was fit to two university counseling center datasets (N=4,679 and N=4,500) of Outcome-Questionnaire-45.2 (OQ) psychotherapy intake data. The bifactor model showed superior fit compared to a unidimensional IRT model. IRT item parameters derived from the bifactor model show that items discriminate well on the primary scale. Items on the OQ's subscales maintain some discrimination ability over and above the primary scale. However, reliability estimates for the subscales, controlling for the primary scale, suggest that clinical use should likely proceed with caution. Item difficulty or severity parameters reflected item content well, in that increased probability of endorsement was found at high levels of distress for items tapping severe symptomatology. Increased probability of endorsement was found at lower levels of distress for items tapping milder symptomatology. Analysis of measurement invariance showed that item parameters hold equally across gender for most OQ items. A subset of items was found to have item parameters non-invariant across gender. Implications for research and practice are discussed, and directions for future work given.

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