Validating the sumi quality of life questionnaire with rasch analysis.

PURPOSE To validate the Sumi Visual Disability Questionnaire (Sumi VDQ) to estimate visual disability in glaucoma patients using Rasch analysis. METHODS A total of 162 glaucoma patients underwent visual field (VF) testing in both eyes (Humphrey 24-2 Swedish Interactive Threshold Algorithm [SITA] standard program). The binocular VF was then calculated using the integrated VF (IVF) method. Visual disability was scored using the Sumi VDQ, which was originally written in Japanese and contains 30 items (questions). Response scale analysis, targeting, and infit statistics associated with Rasch analysis were evaluated. Unidimensionality was analyzed using principal component analysis (PCA). In addition, the correlation between the person parameter obtained with Rasch analysis and the mean of total deviation values (mTD) in the IVF was compared with the correlation between the arithmetic sum of visual disability score and mTD of the IVF. RESULTS All 30 items in the Sumi VDQ showed productive infit values (0.61-1.46). The person parameters distributed between -4.50 and 3.62, while the item difficulty targeting parameters distributed between -0.88 and 2.06. None of the PCA components had eigenvalues whose lower limit of 95% confidence interval (CI) exceeded 2 (0-1.5). There was a significant relationship between person parameter and mTD of IVF (r = -0.78, P < 0.001), which was significantly stronger (Meng-Rosenthal-Rubin method, P = 0.002) than that between arithmetic sum of visual disability score and mTD of IVF (r = -0.61, P < 0.001). CONCLUSIONS The Sumi VDQ has constructive psychometric properties. In particular, the Rasch analysis-derived person parameter appears to be clinically more meaningful than the arithmetic sum of visual disability score.