Abstract
BackgroundOver 2 billion people suffer from vision impairment or blindness globally, and access to validated visual measurement tools in imperative in accurately describing and managing the burden of eye disease. The present study applies contemporary psychometric validation techniques to the widely used 33-item Indian Visual Function Questionnaire (IND-VFQ-33).MethodsWe first estimated the polychoric correlation between each pair of items. Next, an unrotated and oblique Promax rotated factor analysis, item response theory (IRT, using a graded response model (GRM)), and differential item functioning (DIF) testing were applied to the IND-VFQ-33. We subsequently propose a validated IND-VFQ-33 questionnaire after psychometric testing, data reduction, and adjustment.ResultsExploratory unrotated factor analysis identified two factors; one with a particularly high eigenvalue (18.1) and a second with a lower eigenvalue still above our threshold (1.1). A subsequent oblique Promax factor rotation was undertaken for a 2-factor solution, revealing two moderately correlated factors (+ 0.68) with clinically discrete item loadings onto either Factor 1 (21 items; collectively labelled “daily activities”) or Factor 2 (5 items; collectively labelled “bright lights”). IRT confirmed high item discrimination for all remaining items with good separation between difficulty thresholds. We found significant DIF on depression for six items in Factor 1 (all uniform DIF, except item 21 (non-uniform DIF) with no substantive difference in beta thresholds for any item and no substantive difference in expected individual or sum score, by depression at baseline. For Factor 2, only one item demonstrated significant uniform DIF on gender, similarly without major differences in beta thresholds or expected total score between gender at baseline. Consequently, no further item recalibration or reduction was undertaken after IRT and DIF analysis.ConclusionApplying IRT and DIF validation techniques to the IND-VFQ-33 identified 2 discrete factors with 26 uniquely-loading items, clinically representative of difficulty performing daily activities and experiencing difficulty due to bright lights/glare respectively. The proposed modified scale may be useful in evaluating symptomatic disease progression or response to treatment in an Indian population.
Highlights
Over 2 billion people suffer from vision impairment or blindness globally, and access to validated visual measurement tools in imperative in accurately describing and managing the burden of eye disease
The proposed modified scale may be useful in evaluating symptomatic disease progression or response to treatment in an Indian population
Based on lack of fit to the model, they recommended deleting the visual symptoms and psychosocial impact scales. They argued that the general function scale exhibited multidimensionality, and that it should be divided into two scales measuring visual function and vision-related mobility impairments [7]
Summary
Over 2 billion people suffer from vision impairment or blindness globally, and access to validated visual measurement tools in imperative in accurately describing and managing the burden of eye disease. With globally aging populations [5], access to visual-measurement tools validated for use in older populations is imperative in accurately describing and managing the burden of eye disease [6]. The Indian Visual Function Questionnaire (IND-VFQ33) is a 33-question survey originally psychometrically validated in 2004 by Gupta and colleagues on a population of 780 patients in India [6]. Based on lack of fit to the model, they recommended deleting the visual symptoms and psychosocial impact scales They argued that the general function scale exhibited multidimensionality, and that it should be divided into two scales measuring visual function and vision-related mobility impairments [7]
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