Reliability and Validity of Scores from the Foot and Ankle Ability Measure

• Main Authors: Lauren Matheny MPH, Thomas Clanton MD
• Format: Article
• Language: English
• Published: SAGE Publishing 2018-09-01
• Series: Foot & Ankle Orthopaedics
• Online Access: https://doi.org/10.1177/2473011418S00083

Category: Ankle Introduction/Purpose: A commonly used measure of ankle function is the Foot and Ankle Ability Measure (FAAM). To support interpretation of the FAAM, evidence of reliability and validity must be established. Some studies have assessed FAAM scores; however, these studies had small sample sizes, sample characteristics that may limit generalizability, and did not report reliability estimates. These studies were also unable to account for person ability and item difficulty, a unique feature Rasch modeling offers, which is key when attempting to generalize to other populations. The purpose of this study was to determine whether there is evidence of reliability and validity for the FAAM ADL and Sport scales, utilizing the Rasch model, in patients who have undergone surgical intervention for the treatment of an ankle injury. Methods: Evidence of reliability and validity were determined utilizing the Rasch measurement model, a special case of item response theory, which has been used to develop new patient reported outcome measures and improve existing measures. This is a widely used technique that may be used as an alternative to classical test theory due to advantages including generalizability across samples, accounting for response options not equally spaced in terms of ability, and identifying poorly functioning items. The scale of interest is measured in terms of item difficulty and generates estimates of locations of individual items (item difficulty) and ability level along a common interval-level scale (log-odds). To identify misfit items, outfit mean-square (MNSQ) and infit MNSQ statistics were assessed. Infit and outfit MNSQ range from 0 to positive infinity (ideal value of 1.0 means observed variance = expected variance; acceptable value range 0.5 -1.7). Person reliability was also reported (analogous to Cronbach’s a). Results: There were 456 patients included in the study(192 females, 264 males)(average age=47.6 years(18-79). Rasch analysis showed good evidence of reliability for FAAM ADL and FAAM Sport scores (Figure 1). Person reliability was 0.87 for FAAM ADL and 0.89 for FAAM Sport. Outfit MNSQ values for FAAM ADL items 11 (Coming Up On Toes) and 10 (Squatting) were high (2.17, 1.96). Item 19 “Light/Moderate Work” was low(0.48), indicating item redundancy. For FAAM Sport, all outfit values (range 0.67 -1.64) were within the acceptable range. For internal scale validity, infit MNSQ values for FAAM ADL items 11 and 10 were high(2.30, 2.05). All other infit values (range 0.61 -1.48) were within the acceptable range. For FAAM Sport, all infit values (range 0.74 -1.65) were within the acceptable range. Conclusion: This study provides good evidence of reliability for FAAM ADL and Sport scores in a wide range of patients who underwent ankle surgery, which may demonstrate wide clinical applicability. Both scales demonstrated good internal scale validity; however, 3 FAAM ADL items may indicate the need for further scale development for use in a diverse ankle population.