The Occupational Case Analysis Interview and Rating Scale (OCAIRS) was developed based on the Model of Human Occupation with the intention of assessing patients' occupational adaptation. Several studies examining the quality of this instrument have been completed; however, none have discussed the internal validity of the instrument or the appropriateness of the rating scale. The purpose of this study is to validate the internal validity of the OCAIRS and to test the quality of the rating scale. The results indicate that the OCAIRS is a valid measure of occupational adaptation. Each item was shown to have its own rating scale structure, however, all items together still shared the same five-point rating scale.