Structural Validity of the Foot and Ankle Outcome Score for Orthopaedic Pathologies with Rasch Measurement Theory

Background The Foot and Ankle Outcome Score (FAOS) is one of the most frequently used patient-reported outcome measures for foot and ankle conditions. The aim is to test the structural validity of the Finnish version of the FAOS using Rasch Measurement Theory. Methods FAOS scores were obtained from 218 consecutive patients who received operative treatment for foot and ankle conditions. The FAOS data were fitted into the Rasch model and person separation index (PSI) calculated. Results All the five subscales provided good coverage and targeting. Three subscales presented unidimensional structure. After testlet formation, all subscales were unidimensional. All subscales showed sufficient fit to the Rasch model. PSI ranged from 0.73 to 0.94 for the subscales. Conclusions The Finnish version of the FAOS shows acceptable structural validity for assessing complaints among orthopaedic foot and ankle patients.