William Mitchell1,2, Srinivas Marmamula3,4,5,6, Nazlee Zebardast7,8, Weiwen Ng9, Joseph J Locascio10, Thirupathi Kumbam3, Satya Brahmanandam3, Navya Rekha Barrenkala3. 1. Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Boston, USA. william.greig.mitchell@gmail.com. 2. Harvard TH Chan School of Public Health, Harvard University, Boston, USA. william.greig.mitchell@gmail.com. 3. Allen Foster Community Eye Health Research Centre, LV Prasad Eye Institute, Hyderabad, India. 4. Brien Holden Institute of Optometry and Vision Science, LV Prasad Eye Institute, Hyderabad, India. 5. Department of Biotechnology/Wellcome Trust India Alliance, LV Prasad Eye Institute, Hyderabad, India. 6. School of Optometry and Vision Science, University of New South Wales, Kensington, Australia. 7. Massachusetts Eye and Ear Infirmary, Department of Ophthalmology, Boston, USA. 8. Department of Ophthalmology, Harvard Medical School, Boston, USA. 9. University of Minnesota School of Public Health, University of Minnesota, Minneapolis, USA. 10. Department of Neurology, MA General Hospital, Boston, USA.
Abstract
BACKGROUND: 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 present study applies contemporary psychometric validation techniques to the widely used 33-item Indian Visual Function Questionnaire (IND-VFQ-33). METHODS: We 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. RESULTS: Exploratory 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. CONCLUSION: Applying 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.
BACKGROUND: 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 present study applies contemporary psychometric validation techniques to the widely used 33-item Indian Visual Function Questionnaire (IND-VFQ-33). METHODS: We 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. RESULTS: Exploratory 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. CONCLUSION: Applying 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.
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