Psychometric properties and developmental differences in children's ADL item hierarchy -: A study of the WeeFIM® instrument

Objective: To investigate the measurement properties of the WeeFIM((R)) instrument and examine the developmental differences in motor item difficulty in a pediatric inpatient rehabilitation sample. Design: Database approach, retrospective study, using 814 WeeFIM records from 12 facilities. Rasch rating scale analysis was used for all analyses. Data for the complete sample were used to evaluate the dimensionality and item fit of the WeeFIM instrument. Patients were then divided into three age groups, < 3 yrs of age (toddlers), 3-7 yrs of age (preschoolers and kindergartners), and > 7 yrs of age (school-age children), and their data were used to compare the order of motor items by level of difficulty within and across different age groups. Results: Principal component analysis of the WeeFIM suggests distinct motor and cognitive scales. Within the motor scale, bowel and bladder items misfit, suggesting they measure a distinct aspect of function. The order of motor item difficulty, an indicator of construct validity, varies across age groups, suggesting that motor tasks present different challenges to children at different developmental stages: toileting is the most difficult, whereas locomotion is the easiest motor item for toddlers. For children > 3 yrs of age, eating is easiest and stair climbing is most difficult, followed by tub transfer and bathing. Conclusions: Similar to the adult FIM (TM) instrument, the WeeFIM instrument has two distinct dimensions. The motor items form a unidimensional construct with acceptable measurement properties. Developmental differences in motor task mastery among children with disabilities are assumed but rarely tested. As evidenced by the age-specific item hierarchies found in this study, developmental differences among children with disabilities mimic that of children without disabilities.