Are standard instruments valid for the assessment of quality of life and symptoms in glaucoma?

Abstract

PURPOSE: To determine if the impact of Visual Impairment Instrument (IVI) and Glaucoma Symptom Scale (GSS) are valid instruments to assess participation in daily living and ophthalmic complaints, respectively, in a glaucoma population. METHODS: Patients with glaucoma were recruited from private and public clinics and completed the IVI and GSS questionnaires. The two scales were assessed for fit to the Rasch model. Unidimensionality, individual item and person fit to the model, response category performance (how respondents differentiate between the response options), differential item functioning (how subgroups, despite equal levels of the underlying trait, respond differently to an individu..