Rasch analysis of a new stroke-specific outcome scale: the Stroke Impact Scale

Abstract

Objectives: To assess multiple psychometric characteristics of a new stroke outcome measure, the Stroke Impact Scale (SIS), using Rasch analysis, and to identify and remove misfitting items from the 8 domains that comprise the SIS.

Design: Secondary analysis of 3-month outcomes for the Glycine Antagonist in Neuroprotection (GAIN) Americas randomized stroke trial.

Setting: A multicenter randomized trial performed in 132 centers in the United States and Canada.

Participants: A total of 696 individuals with stroke who were community-dwelling and independent prior to acute stroke.

Interventions: Not applicable.

Main outcome measures: Rasch analysis was performed using WINSTEPS, version 3.31, to evaluate 4 psychometric characteristics of the SIS: (1) unidimensionality or fit (the extent to which items measure a single construct), (2) targeting (the extent to which the items are of appropriate difficulty for the sample), (3) item difficulty (the ordering of items from least to most difficult to perform), and (4) separation (the extent to which the items distinguish distinct levels of functioning within the sample).

Results: (1) Within each domain, most of the items measured a single construct. Only 3 items misfit the constructs and were deleted ("add and subtract numbers," "get up from a chair," "feel emotionally connected") and 2 items ("handle money," "manage money") misfit the combined physical domain. These items were deleted to create SIS, version 3.0. (2) Overall, the items are well targeted to the sample. The physical and participation domains have a wide range of items that capture difficulties that most individuals with stroke experience in physical and role functions, while the memory, emotion, and communication domains include items that capture limitations in the most impaired patients. (3) The order of items from less to more difficult was clinically meaningful. (4) The individual physical domains differentiated at least 3 (high, average, low) levels of functioning and the composite physical domain differentiated more than 4 levels of functioning. However, because difficulties with communication, memory, and emotion were not as frequently reported and difficulties with hand function were more frequently reported, these domains only differentiated 2 (high, low) to 3 (high, average, low) strata of patients. Time from stroke onset to administration of the SIS had little effect on item functioning.

Conclusion: Rasch analysis further established the validity of the SIS. The domains are unidimensional, the items have an excellent range of difficulty, and the domain scores differentiated patients into multiple strata. The activities of daily living/instrumental activities of daily living, mobility, strength, composite physical, and participation domains have the most robust psychometric characteristics. The composite physical domain is most able to discriminate difficulty in function in individuals after stroke, while the communication, memory, and emotion domain items only capture limitations in function in the more impaired groups of patients.