Assessing internal construct validity of DAPSA and DAPSA28 in psoriatic arthritis: a European observational study using confirmatory factor analysis and additional psychometric testing.

Differential item functioning The analyses of DAPSA DIF based on baseline scores revealed highly significant evidence of DIF with respect to country (see online supplemental table S6 ).

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Fit indices For DAPSA and DAPSA28, CFA models testing configural invariance—which checks whether the index functions the same way across groups—showed good fit at baseline, although the p-value for DAPSA28 was borderline significant.

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