Scaling Properties of the Subjective Index of Physical and Social Outcome After Stroke in a Study Population Unselected by Age

Abstract

ObjectiveTo investigate the scaling properties of the Subjective Index of Physical and Social Outcome (SIPSO) after stroke in survivors to hospital discharge, unselected by age. DesignFactor, Mokken, and Rasch analyses of the SIPSO using data from a prospective observational cohort study. SettingThree acute care hospitals. ParticipantsConsecutive admissions (N=312) with acute stroke, unselected by age. InterventionsNot applicable. Main Outcome MeasuresPatient- or proxy-reported SIPSO, collected by postal survey 6 months after stroke. ResultsComplete 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 (χ2 test for item-trait interaction, χ2=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 (χ2=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 (χ2=5.21; P=.27) and targeting to the latent trait were good, and thresholds ordered. Separation reliability was poor (Person-Separation Index, .67). ConclusionsThe 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.