Many-faceted rasch calibration of the foot function index-revised short form

The Foot Function Index Revised Short version (FFI-RS) is a foot- and ankle- patient-reported outcome measure (PROM), developed from the Foot Function Index (FFI). Previous studies, estimating item parameters and multidimensional properties, have limitations properly establishing the measurement properties of the FFI-RS. A multi-faceted Rasch analysis with a larger sample would allow for a more robust validation approach to improve the clinical interpretation of the FFI-RS using a multidimensional perspective. Therefore, the purpose of this study was to assess the psychometric properties of the FFI-RS as a PROM of foot function.

A total of 2184 patients with foot pathology who completed the FFI-RS were included in the data. Data were extracted from the cloud-based orthopedic and sports medicine global registry Surgical Outcome System (SOS). The psychometric properties of the FFI-RS were assessed using a many-faceted Rasch analysis that included model-data fit, rating scale function, item-person map (distribution of item difficulty and person ability), and item difficulty of the subscale.

Two misfit items were discovered and deleted; 32-items from the original FFI-RS were retained. The 4-item Likert scale functioned effectively and item difficulty (-0.58 to 1.48), subscale difficulty (-0.58 to 1.15), and person's foot function (-6.62 to 6.24) had wide distributions.

Many-faceted Rasch analysis revealed the FFI-RS had sound psychometric properties using the many-faceted Ranch analysis and retained 32 of the original items. Clinicians and researchers should consider weaknesses identified with items in the 'Difficulty" subscale and future work should be conducted to modify or develop items that will more accurately evaluate a wide range of foot function levels.