Rasch analysis of the activities-specific balance confidence scale in older adults seeking outpatient rehabilitation services

BACKGROUND: The Activities-specific Balance Confidence (ABC) scale measures confidence in performing various ambulatory activities without falling or experiencing a sense of unsteadiness. OBJECTIVES: This study (1) examined the ABC scale (0-100) using Rasch analysis, (2) assessed statistically reliable change, and (3) developed a functional staging to guide clinical interpretation of a patient's improvement. METHODS: The authors examined rating-scale structure, item difficulty hierarchy, item fit, person-item match, separation index, differential item functioning, test precision, and unidimensionality. Additionally, this cross-sectional study of 5012 older patients seeking outpatient rehabilitation therapy in 123 clinics estimated the minimal detectable change and developed a functional staging. RESULTS: The item "walk outside on icy sidewalks" was the most difficult item, while the item "reach for a small can off a shelf at eye level" was the easiest item. Overall, average patient ability estimates of 56.2 ± 20.3 were slightly higher than the average item difficulty estimates of 45.9 ± 7.8. With a separation index equal to 3.65, the ABC scale items can differentiate individuals into 5.2 statistically distinct strata. Most ABC scale items were free of differential item functioning. For example, "walk outside on icy sidewalks" was easier for patients who were underweight. Results supported unidi-mensionality of the ABC scale, with the first factor explaining 77% of the total variance. The estimated minimal detectable change was 15 points. The authors provided an example of functional staging application. CONCLUSION: Results supported sound psychometric properties and clinical usage of the ABC scale for older adults seeking outpatient rehabilitation therapy.