| Summary: | Abstract Background The Patient Health Questionnaire-8 (PHQ-8) is a screening questionnaire of depressive symptoms. However, it is unknown whether it is equivalent across time and between groups of individuals. The aim of our paper was to test whether the PHQ-8 has the same meaning in two groups of individuals over time. Methods Primary care patients were proactively recruited from three German cities. PHQ-8 data from a baseline assessment (n = 588), two assessments during the intervention (n = 246/225), and a six (n = 437) and 12 months (n = 447) follow-up assessment were first used to examine the factor structure of the PHQ-8 by confirmatory factor analysis (CFA). The best fitting factor solution was then used to test longitudinal invariance across time and between intervention and control group by Multiple Group CFA. Results A two-factor structure consistently showed the best model fit. Only configural longitudinal invariance was evidenced when the baseline assessment was included in the analysis. Without the baseline assessment, strict longitudinal invariance was shown across the intervention and the follow-up assessments. Scalar invariance was established between the intervention and control group for the baseline assessment and strict invariance between groups and across the 6- and 12-month follow-up assessments. Conclusions The lack of longitudinal invariance might be attributed to various differences between the baseline assessments and all following assessments, e.g., assessment mode (iPad vs telephone), potential changes in symptom perception, and setting. Trial registration DRKS0001163 5, date of trial registration: 20.01.2017; DRKS00011637 , date of trial registration: 25.01.2017.
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