Measurement precision of the clinician administered PTSD scale (CAPS): a RASCH model analysis.

The Clinician Administered PTSD Scale (CAPS), originally developed as a diagnostic tool, is frequently used to evaluate treatment responses. Defining a case and measuring symptom changes are different processes that require different attributes for the instrument. Measuring symptom changes requires precision in measurement. Using the Rasch rating scale model, we evaluated this instrument for construct validity in a veteran sample. The distribution of the veteran measures did not align with the distribution of the item measures in the CAPS instrument. Separate analysis of the CAPS Frequency subscale and Intensity subscale were conducted. The Frequency subscale produced measures that encompassed the level of severity found in the veteran sample. Items from this instrument can be used to develop an equal interval scale to provide precise measurements for treatment evaluations and to identify clinical cut points for diagnostic purposes.