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Brenda W. Gillespie, David C. Musch, Leslie M. Niziol, Nancy K. Janz; Estimating Minimally Important Differences for Two Vision-Specific Quality of Life Measures. Invest. Ophthalmol. Vis. Sci. 2014;55(7):4206-4212. doi: 10.1167/iovs.13-13683.
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To estimate minimally important differences (MIDs) for the Visual Activities Questionnaire (VAQ) and the National Eye Institute-Visual Function Questionnaire (NEI-VFQ).
A total of 607 subjects with newly-diagnosed open-angle glaucoma (OAG) was enrolled in the Collaborative Initial Glaucoma Treatment Study (CIGTS) and randomized to initial treatment with medications or surgery. Subjects underwent an ophthalmic examination and telephone-administered quality of life (QOL) interview before randomization and every six months thereafter. The VAQ and NEI-VFQ were used to assess participants' perceptions of their visual function. Clinical measures included the mean deviation (MD) from Humphrey 24-2 full threshold visual field (VF) testing, and best-corrected visual acuity (VA) measured using the Early Treatment Diabetic Retinopathy Study (ETDRS) protocol. Anchor-based (using MD and VA) and distribution-based methods were used to estimate MIDs.
Anchor-based cross-sectional analyses at 66 months follow-up found a 10-letter increment in better eye VA corresponded to MIDs of 5.2 units for VAQ and 3.8 units for NEI-VFQ total scores. A 3-dB increment in the better eye MD yielded MIDs of 2.6 and 2.3 units for the same two questionnaires. In longitudinal analyses, MIDs for the VAQ were 3.2 units for a 10-letter change of VA and 3.4 units for a 3-dB change in the MD. Distribution-based MIDs were larger.
A range of MIDs for the VAQ (2.6–6.5 units) and NEI-VFQ (2.3–3.8 units) was found. Although similar in magnitude, MIDs were sensitive to the MID estimation method, the anchor chosen, and differences between questionnaires. (ClinicalTrials.gov number, NCT00000149.)
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