Development of a Flexilevel Scale for use with computer-adaptive testing for assessing shoulder function

In a 5-year study, a self-report measure of shoulder function--the Flexilevel Scale of Shoulder Function (FLEX-SF)--was developed by use of item response theory. A large pool of candidate items (N = 68) was developed. A questionnaire that included the 68 items, another scale of shoulder function, and clinical and demographic questions were administered to 400 persons with shoulder complaints. Patients' responses to the 68 items were calibrated by use of Andrich's rating scale model. Thirty-three items were selected from the pool and subdivided into three overlapping testlets targeting low, medium, and high shoulder function. A table translates raw scores on testlets to a common mathematical metric. The validity and reliability of the FLEX-SF was evaluated in a longitudinal study of 199 patients. The FLEX-SF scores were highly reliable and exhibited excellent validity (including responsiveness). We report on a simulation of a computer-adaptive test of shoulder function. This simulation is based on the developmental items we tested for use in the FLEX-SF. The results indicate that greater measurement efficiency can be achieved with a computer-adaptive test format.