Structural validity of the Trunk Assessment Scale for Spinal Cord Injury (TASS) with Rasch analysis for individuals with spinal cord disorders

Abstract

Objective To confirm the structural validity of the Trunk Assessment Scale for Spinal Cord Injury (TASS). Participants and Methods We evaluated 104 Japanese individuals with a spinal cord injury (SCI) (age 63.5 ± 12.2 years; 64 with tetraplegia) with the TASS 1−3 times. We conducted a Rasch analysis to assess the TASS’ unidimensionality, fit statistics, category probability curve, ceiling/floor effects, local independence, reliability, and difference item function (DIF). Results The TASS was observed to be a unidimensional and highly reproducible scale of item difficulty hierarchy that sufficiently identifies the superiority of the examinee's ability. The TASS was easy for the participants of this study. One TASS item was a misfit based on the infit and outfit mean square; another item also showed a DIF contrast for age. Several items were found to require a synthesis or modification of the content. The TASS showed a floor effect, and most of the non-scorers were individuals with a complete SCI. Conclusion Our findings clarify the structural validity of the TASS, and our analyses revealed that the TASS includes an unfitness item and was less challenging for individuals with SCIs. The improvements suggested by these results provide important information for modifying the TASS to a more useful instrument.