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Vijaya K. Gothwal, Shailaja P. Reddy, Rebecca Sumalini, Seelam Bharani, Deepak K. Bagga; National Eye Institute Visual Function Questionnaire or Indian Vision Function Questionnaire for Visually Impaired: A Conundrum. Invest. Ophthalmol. Vis. Sci. 2012;53(8):4730-4738. doi: 10.1167/iovs.11-8776.
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Both the long form visual functioning scale (LFVFS39) and visual functioning scale (VFS) are measures of visual functioning (VF) that represent the Rasch-scaled versions of the NEI-VFQ39 and the Indian vision function questionnaire (IND-VFQ), respectively. The objectives of this study were to investigate if the 15-item LFVFS39 and 13-item VFS of the IND-VFQ meet the assumptions of the Rasch model and measure the same construct, VF, in an Indian visually impaired (VI) population.
Data from 120 VI adults administered both instruments concurrently, were fitted to the Rasch measurement model to demonstrate that each instrument satisfies the assumptions of the model (including unidimensionality by principal components analysis); and both instruments can be cocalibrated onto a single underlying continuum of VF.
Both instruments required category reorganization for optimal rating scale functioning and possessed similar measurement precision (person separation = 2.76). Separate analysis of each instrument (eigenvalues, 2.3 and 1.9 for LFVFS39 and VFS of IND-VFQ, respectively) and the pooled 28-item analyses (eigenvalue, 2.8) satisfied the assumptions of the Rasch model, including unidimensionality. Furthermore, all items fit in the separate and pooled analyses. Separate item and person measures for each instrument correlated strongly with estimates from the pooled data (r > 0.9 for all, P < 0.0001).
Both the LFVFS39 and VFS of the IND-VFQ measure the same construct, VF, and with equal measurement precision in an Indian VI population. Both instruments can be calibrated onto a single metric, thereby, enabling a comparison of their measurement range of VF.
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