Visual disability assessment: valid measurement of activity limitation and mobility in cataract patients.

AIM: The Visual Disability Assessment (VDA), a questionnaire for measuring the impact of cataract on visual functioning, was developed using classical test theory. Since this approach is limited, our aim was to further investigate the psychometric properties of the VDA using Rasch analysis. METHODS: 613 patients from the Flinders Medical Centre cataract surgery self-administered the VDA. Psychometric properties investigated for the overall VDA and each subscale included: measurement of a single construct (unidimensionality), item fit to the construct, reliable discrimination between strata of patient ability (person separation) and targeting of item difficulty to person ability...