Does the Five Facet Mindfulness Questionnaire measure the same constructs before and after ACT-based treatment? Examination of the longitudinal measurement invariance of the 24-item FFMQ-SF

Violations of longitudinal measurement invariance (LMI) suggest “response shifts” in individuals’ understanding and appraisal of items over time and result in misinterpretation of observed changes in outcome measures. These concerns are crucial to the validity of outcome measures and are particularly relevant in examining the effectiveness of mindfulness and other process-based interventions. In the current report from the Rhode Island Methods to Improve Diagnostic Assessment and Services (MIDAS) project, we examine the LMI of the 24-item short form of the Five Factor Mindfulness Questionnaire from intake to discharge in a large, diagnostically heterogeneous sample of psychiatric patients (n = 2928) participating in an acceptance and commitment therapy-based partial hospitalization program. The original five-factor structure required correlated error terms for items with similar wording (Nonjudging of Experience and Describe scales) to achieve acceptable fit. Following these modifications, we found evidence of configural and weak invariance for all scales. We found evidence for strong invariance for Nonjudging of Experience, Nonreactivity to Inner Experiences, Describe, and Observing, but not for Acting with Awareness. Strict invariance was not supported. Results highlight the challenges of measuring constructs involving overt metacognitive processes via self-report. Implications for scale development and clinical practice are discussed.