The Visual Functioning Questionnaire (VFQ-25) is one of the most widely used measures of vision-related quality of life. However, the questionnaire does not meet some psychometric quality criteria. The objectives of this study were first to obtain the factor structure of the VFQ-25, and second, to obtain interval scales by Rasch analysis.
The questionnaire was administered to 129 visually impaired adults (mean age 42.1 years; range 21 to 67 years). First, the items of the VFQ-25 were subjected to an exploratory factor analysis with Promax rotation. Next, we performed a separate Rasch analysis on each factor. We examined step thresholds and goodness of fit statistics of the items. Finally, we examined differential item functioning.
Factor analysis indicated four factors: Near Activities, Distance Activities and Mobility, Mental Health and Dependency, and Pain and Discomfort. They accounted for 46.37% of the total variance. Most items showed some degree of disordering. After collapsing response categories, all items showed ordered thresholds. The Near Activities domain showed excellent fit, whereas the Distance Activities and Mobility domain, the Mental Health and Dependency domain, and the Pain and Discomfort domain had an unsatisfactory fit. There were two items showing uniform differential item functioning.
The four-factor structure of the VFQ-25 largely confirms the structure of the questionnaire. However, the results of this study suggest that modifications of the original VFQ-25 structure are necessary.