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Gislin Dagnelie, Pamela E. Jeter, Olukemi Adeyemo, PLoVR Study Group; Optimizing the ULV-VFQ for Clinical Use Through Item Set Reduction: Psychometric Properties and Trade-Offs. Trans. Vis. Sci. Tech. 2017;6(3):12. doi: https://doi.org/10.1167/tvst.6.3.12.
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We examine the dimensionality of the 150-item visual functioning questionnaire for individuals with ultralow vision (ULV-VFQ) and develop representative abbreviated versions, facilitating clinical use, while retaining compatibility with a 17-item performance assessment.
Subsets with 50 and 23 items covering the full difficulty range were selected, with evenly spaced item measures (IMs) and good representation of visual aspects and functional domains. Person measures (PMs) for the anchored subsets were derived through Rasch analysis of data from 80 respondents.
Fit statistics for the reduced item sets were similar to those for the full set, with reliabilities at or above 95%. Mean PMs in the reduced sets were within 0.8 standard errors (SEs) of those in the full set. SEs of the PMs increased from the SE for 150 items, roughly in inverse proportion with the square root of the set size. Unexplained variance levels (24%–27%) and variance of the first unexplained factor (3.3%–3.9%) were close to those (30% and 2.6%) for 150 items. Differential item functions for omitted items were negligible. Aspects and domains are adequately represented in the reduced sets.
Self-reported visual ability can be measured accurately using appropriately chosen anchored subsets of the ULV-VFQ. Functional ability of individuals with ULV is characterized adequately by a single dimension.
The ULV-VFQ50 and ULV-VFQ23, using anchored IMs from the 150-item ULV-VFQ, provide an efficient and reliable self-report assessment of visual ability in individuals whose visual impairment is too severe for assessment with VFQs currently in use.
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