An IRT validation of the Affective Self Rating Scale.

BACKGROUND: The Affective Self Rating Scale (AS-18) is intended for the use in bipolar outpatients. It includes subscales for the rating of depressive and manic-type symptoms. It has previously been validated using methods from Classical Test Theory. AIMS: The aim of this study was to evaluate the psychometric properties of the AS-18 when used at an outpatient clinic for patients with bipolar disorder at routine visits, and to analyze the potential for improvement of the scale. METHODS: 231 patients with mainly bipolar I disorder doing ratings on routine visits at an affective disorder outpatient clinic were included. Ratings were analyzed using the Mokken non-parametric and the Rasch parametric model statistical methods. RESULTS: In the Mokken analysis, both subscales of the AS-18 showed a strong ability to rank respondents according to their total score and all items contributed adequately to the measurement. In the Rasch model, there were no indications of disturbing influence from secondary dimensions in the subscales. The depression subscale had the capacity reliably to separate the sample in at most three levels and the mania subscale in two levels. The limited capacity to separate respondents can mainly be explained by a lack of items reflecting lower levels of depressive and manic symptoms. CONCLUSIONS: AS-18 has good basic psychometric properties for use of rating of symptoms in bipolar I patients at routine visits, but there is also room for improvement. Item Response Theory (IRT) methods are suitable tools for evaluation and construction of rating scales.