Applying psychometric criteria to functional assessment in medical rehabilitation: II. Defining interval measures

The use of functional assessment total scores in arithmetic operations has proliferated despite the lack of evidence supporting their use as interval measures of patient ability. Such evidence is minimally necessary to assure the validity of functional assessment total scores for clinical and management decision-making. Two requirements of interval measurement are explained and a set of Rasch analyses of 5,500 assessments using the Patient Evaluation and Conference System (PECS) are presented. The analyses were performed to determine the extent to which four item subsets identified in a previously reported factor analysis of the PECS comprise interval measures of functional independence status. Results indicate that the PECS scales meet these requirements to varying degrees. The analyses also identify areas in which measurement quality can be improved.