A multicenter international study on the Spinal Cord Independence Measure, version III : Rasch psychometric validation

Background: A third version of the Spinal Cord Independence Measure (SCIM III), made up of three subscales, was formulated following comments by experts from several countries and Rasch analysis performed on the previous version. Objective: To examine the validity, reliability, and usefulness of SCIM III using Rasch analysis. Design: Multicenter cohort study. Setting: Thirteen spinal cord units in six countries from North America, Europe, and the Middle-East. Subjects: 425 patients with spinal cord lesions (SCL). Interventions: SCIM III assessments by professional staff members. Rasch analysis of admission scores. Main outcome measures: SCIM III subscale match between the distribution of item difficulty grades and the patient ability measurements; reliability of patient ability measures; fit of data to Rasch model requirements; unidimensionality of each subscale; hierarchical ordering of categories within items; differential item functioning across classes of patients and across countries. Results: Results supported the compatibility of the SCIM subscales with the stringent Rasch requirements. Average infit mean-square indices were 0.79-1.06; statistically distinct strata of abilities were 3 to 4; most thresholds between adjacent categories were properly ordered; item hierarchy was stable across most of the clinical subgroups and across countries. In a few items, however, misfit or category threshold disordering were found. Conclusions: The scores of each SCIM III subscale appear as a reliable and useful quantitative representation of a specific construct of independence after SCL. This justifies the use of SCIM in clinical research, including cross-cultural trials. The results also suggest that there is merit in further refining the scale.