Using Rasch analysis to revisit the validity of the Cataract TyPE Spec instrument for measuring cataract surgery outcomes

Abstract

Purpose: To assess the psychometric properties of the Cataract TyPE Specification (Spec) questionnaire using the Rasch model. Setting: Flinders Medical Centre, Adelaide, Australia. Methods: The 12-item Cataract TyPE Spec questionnaire was self-administered to patients drawn from the cataract surgery waiting list. The questionnaire and its 5 subscales were assessed for fit to the Rasch model. Response category performance, item-fit targeting, unidimensionality (using principal components analysis), and differential item functioning were assessed. A shortened version (11 items) was tested for criterion validity by determining correlation with a global rating of vision question. Results: Two hu..