Investigating the internal validity of the Trunk Impairment Scale (TIS) using Rasch analysis: the TIS 2.0.

PURPOSE: To examine the internal validity of the static sitting balance, dynamic sitting balance, and coordination subscales of the Trunk Impairment Scale (TIS), a reliable and valid scale measuring trunk performance and sitting balance in people after stroke.
METHOD: A total of 162 people after stroke were included in the study. Participants were recruited from an acute unit and in- and out- patient rehabilitation setting. To examine internal validity of the subscales of the TIS, we conducted a Rasch analysis by means of the Partial Credit Model. For each subscale, we examined whether the distribution of scores fitted the theoretical Rasch model.
RESULTS: The first item of the static sitting balance subscale had to be removed since it had a large ceiling effect. The remaining static sitting balance subscale did not fit the Rasch model (Chi-square = 7.03, p < 0.0001 with Bonferroni adjusted p-level = 0.01). Both the dynamic sitting balance (Chi-square = 42.65, p = 0.0052 with Bonferroni adjusted p-level = 0.005) and coordination subscales (Chi-square = 7.87, p = 0.4461 with Bonferroni adjusted p-level = 0.01) fitted the Rasch model.
CONCLUSIONS: Internal validity of the dynamic sitting balance and coordination subscales was confirmed. Based on our results, we present the TIS, version 2.0 (TIS 2.0).