Feasibility of generic, short, and easy-to-use assessment of psychological distress during psychotherapy: Longitudinal measurement invariance of CORE-10 and -OM.

While the CORE-10 inventory for Clinical Outcome Routine Evaluation has shown good psychometric properties in cross-sectional assessment, the feasibility of generic, short, and easy-to-use longitudinal assessment of a broadband construct such as the targeted "general psychological distress" remains to be psychometrically demonstrated. Therefore, we investigated longitudinal measurement invariance (LMI) of CORE-10. For comprehensiveness, we also analyzed its parent inventory, CORE-OM. We investigated the LMI of pre- and post-therapy CORE-10 and -OM assessments in a naturalistic treatment register of 1715 patients' short psychotherapies, testing whether the construct of "psychological distress" remained the same across the treatments. We observed good psychometric properties and no violations of LMI for CORE-10 in chi-squared equivalence tests, nor in effect-size-based evaluations. Only the highly sensitive chi-squared difference tests detected LMI violations but these had little practical influence. The CORE-OM data did not fit well with factorial models but this was cross-sectional rather than a genuinely longitudinal (LMI-related) issue. CORE-10 appeared a structurally valid measure of general psychological distress and suitable for longitudinal assessment, whereas the CORE-OM had a less clear factorial structure. Regarding psychometrics, these findings support the use of CORE-10 in longitudinal assessment during psychotherapy and do not support CORE-OM.