Psychometric evaluation and Rasch analyses of the German Overall Anxiety Severity and Impairment Scale (OASIS-D).

The Overall Anxiety Severity and Impairment Scale (OASIS) is a 5-item self-report measure that captures symptoms of anxiety and associated functional impairments. This study evaluates a German version (OASIS-D) that was administered to a convenience sample of 1398 primary care patients of whom 419 were diagnosed with panic disorder with/without agoraphobia. Psychometric properties were analyzed using classical test theory as well as probabilistic test theory. Factor analyses suggested a unitary (latent) factor structure. The internal consistency was good to excellent. Convergent as well as discriminant validity with other self-report measures was found. A sum score (range 0-20) of ≥ 8 emerged as optimal cut-score for screening purposes. A difference score of ≥ 5 was indicative of reliable individual change. A Rasch analysis of local item independence suggested response dependency between the first two items. Rasch analyses of measurement invariance detected noninvariant subgroups associated with age and gender. Analyses of validity and optimal cut-off score were solely based on self-report measures, which may have introduced method effects. In sum, the findings support the transcultural validity of the OASIS and indicate its applicability to naturalistic primary care settings. Caution is warranted when using the scale to compare groups that differ in age or gender.