Scaling properties of the subjective index of physical and social outcome after stroke in a study population unselected by age

Elizabeth A Teale, Theresa M Munyombwe, John B Young
Archives of Physical Medicine and Rehabilitation 2013, 94 (12): 2448-55

OBJECTIVE: To investigate the scaling properties of the Subjective Index of Physical and Social Outcome (SIPSO) after stroke in survivors to hospital discharge, unselected by age.

DESIGN: Factor, Mokken, and Rasch analyses of the SIPSO using data from a prospective observational cohort study.

SETTING: Three acute care hospitals.

PARTICIPANTS: Consecutive admissions (N=312) with acute stroke, unselected by age.

INTERVENTIONS: Not applicable.

MAIN OUTCOME MEASURES: Patient- or proxy-reported SIPSO, collected by postal survey 6 months after stroke.

RESULTS: Complete SIPSO questionnaires were returned by 166 of 268 survivors (median age, 72y; interquartile range, 66-81y). Factor and Mokken analyses supported both 1- and 2-factor solutions. Fit to the Rasch model for the 10-item scale was poor (χ(2) test for item-trait interaction, χ(2)=69.6; P<.001). Differential item functioning by sex and age was demonstrated for the physical subscore and was dealt with through the creation of 2 super items, resulting in a good fit to the Rasch model (χ(2)=2.35; P=.67), ordered thresholds, good targeting to the latent trait, and reasonable separation reliability (Person-Separation Index, 0.8). For the social subscore, no differential item functioning was demonstrated by age or sex. Local dependence was dealt with through the creation of 2 super items. Thereafter, fit to the Rasch model (χ(2)=5.21; P=.27) and targeting to the latent trait were good, and thresholds ordered. Separation reliability was poor (Person-Separation Index, .67).

CONCLUSIONS: The 10-item SIPSO is a valid ordinal scale in a population including older stroke survivors. A physical and social subscale structure is also supported. Subscales can be manipulated to fit the Rasch model, and a conversion table for conversion to an interval scale is provided. The social subscore has poor separation reliability, limiting its use in older stroke survivors.

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