The Outcome Questionnaire-45 (OQ-45) is a 45-item instrument designed to be used by psychotherapists to track their clients' distress over time. The OQ-45 is composed of three factors: symptomatic distress, interpersonal relations, and social role performance. Numerous researchers have attempted to replicate this intended three-factor structure in their own data, only to find poor fit. Attempts to find a factor structure that does show adequate fit have been met with mixed, but generally poor, results. Additionally, very little work has been done to establish that the OQ-45 exhibits sufficient longitudinal measurement invariance to allow comparison of OQ-45 scores over time. Notwithstanding these known issues regarding the fit of the OQ-45, it has been adopted widely in many countries and translated into several dozen languages. This study sought to identify a factor structure of the OQ-45 that did exhibit longitudinal measurement invariance. Using a sample of 7,751 clients who made 56,353 visits to Brigham Young University's Counseling and Psychological Services between 1996 and 2017, three factor structures were analyzed using Mplus 8.2 through confirmatory factor analysis: (a) single-factor, (b) intended three-factor, and (c) bifactor models. The bifactor model fit the data best, as determined by standard fit statistics (CFI, TLI, RMSEA, SRMR). However, this bifactor model still had inadequate fit. At this point, exploratory structural equation modeling (ESEM) using target rotation was applied to the bifactor model. This ESEM bifactor model had a dominant general factor and did have good fit to the data. Having selected the ESEM bifactor model, it was then tested to see if it showed longitudinal measurement invariance over five time points (the initial OQ-45 score at the intake appointment, followed by four subsequent appointments). The OQ-45 items were treated as categorical and analyzed using the WLSMV estimator. Four time sequences were examined for configural, metric, and scalar longitudinal invariance: Time 1 to Time 2, Time 1 to Time 3 (inclusive of Time 2), Time 1 to Time 4 (inclusive of Times 2 and 3), and Time 1 to Time 5 (inclusive of Times 2, 3, and 4). The OQ-45, when modeled as an ESEM bifactor model, does exhibit scalar longitudinal measurement invariance. Using a new method developed by Clark (2020), ΔSRMR between adjacent models (configural to metric, metric to scalar) were all below his recommended guideline of .01. This is the first study to find a good fitting model of the OQ- 45 that can be used to assess changes in clients' psychological functioning over time. Total OQ- 45 scores can continue to be used by therapists to monitor their patients with confidence in its longitudinal psychometric properties.



College and Department

David O. McKay School of Education; Educational Inquiry, Measurement, and Evaluation



Date Submitted


Document Type





outcome measures, bifactor models, structural equation models, confirmatory factor analysis, longitudinal measurement invariance



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Education Commons