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. RESULTS The VDA discriminated five strata of patient ability. However, seven mobility items constituted a second dimension and formed a valid separate scale. Sequestration of these items resulted in a unidimensional 11-item measure of activity limitation. Both the mobility and activity limitation scales had acceptable person separation and neither contained misfitting items. Targeting was suboptimal for mobility (-2.12 logits) but good for activity limitation (-0.72). The subscales also satisfied the requirements of the Rasch measurement model. CONCLUSIONS The Rasch-scaled VDA effectively measures two separate constructs: mobility and activity limitation (with two subscales). Its good psychometric properties make it suitable for measuring cataract surgery outcomes.