The Impact of Vision Impairment questionnaire: an assessment of its domain structure using confirmatory factor analysis and Rasch analysis

PURPOSE. To assess and validate the subscale structure of the 28-item Impact of Visual Impairment (IVI) Scale by using confirmatory factor analysis (CFA) and Rasch analysis for use as an outcome measure. METHODS. Three hundred nineteen participants completed the IVI questionnaire, and the responses then were subjected to Rasch analysis by RUMM2020 software. With the person estimates for each item, CFA was used to assess two hypothesized structures: three-and four-factor models. The subscales of the model with the best fit were then further validated by Rasch analysis. RESULTS. CFA supported a three-factor model that included items from the emotional well-being, reading and accessing information, and mobility and independence subscales. Almost all the selected goodness-of-fit statistics for the three-factor model were better than the recommended values. The factor loadings of the items on their respective domains were all statistically significant (P 0.001) and ranged between 0.54 and 0.81. The three subscales individually fitted the Rasch model according to the item‐trait interaction test (mobility and independence 2 [df] 45.9 [44], P 0.39; emotional wellbeing 28.4 [32], P 0.65; and reading and accessing information 43.5 [36], P 0.18). The item-fit residuals values of the three subscales were 2.5 and showed mean and standard deviations approximating 0 and 1, respectively. The internal consistency reliability of the subscales () was substantial, ranging between 0.89 and 0.91. CONCLUSIONS. An examination of the IVI dimension confirmed a three-subscale structure that displays interval measurement characteristics likely to provide a valid and reliable assessment of restriction of participation. The findings provide an opportunity for a more detailed measurement of the effects of different types of low-vision rehabilitation programs. (Invest Ophthalmol Vis Sci. 2007;48:1001‐1006) DOI:10.1167/iovs.06-0361 T he Impact of Vision Impairment (IVI) questionnaire was designed to assess participation in daily activities and determine the outcome of low-vision rehabilitation on quality of life in people with low vision. 1‐5 The IVI has been validated and shows good discriminative ability and reliability, with consistent results found between different forms of administration. 1 It has a good range of items and, as opposed to most vision-specific questionnaires which typically assess visual functioning, has been designed to assess restriction of participation in daily living as well as to provide a tool to determine the effectiveness of low-vision rehabilitation. The original IVI questionnaire contained 32 items grouped under five domains of participation: leisure and work, consumer and social interaction, household and personal care, mobility, and emotional reaction to vision loss. The grouping of items within domains is important because they can form subscales that allow for the assessment of intervention at more specific levels. This is particularly relevant to low-vision care, as management is typically undertaken on a task-specific basis, and so it is possible that real gains in one or two areas may be obscured within a large scale assessing overall performance. Subscale measurements of outcome provide a more detailed insight into the effectiveness aspects of low-vision rehabilitation. In the initial validation of the IVI questionnaire, principal components analysis (PCA) was used to explore the underlying structure of the scale, but no Rasch analysis was undertaken, making the findings essentially infer