Abstract
BackgroundFor the Low Vision Quality Of Life questionnaire (LVQOL) it is unknown whether the psychometric properties are satisfactory when an item response theory (IRT) perspective is considered. This study evaluates some essential psychometric properties of the LVQOL questionnaire in an IRT model, and investigates differential item functioning (DIF).MethodsCross-sectional data were used from an observational study among visually-impaired patients (n = 296). Calibration was performed for every dimension of the LVQOL in the graded response model. Item goodness-of-fit was assessed with the S-X2-test. DIF was assessed on relevant background variables (i.e. age, gender, visual acuity, eye condition, rehabilitation type and administration type) with likelihood-ratio tests for DIF. The magnitude of DIF was interpreted by assessing the largest difference in expected scores between subgroups. Measurement precision was assessed by presenting test information curves; reliability with the index of subject separation.ResultsAll items of the LVQOL dimensions fitted the model. There was significant DIF on several items. For two items the maximum difference between expected scores exceeded one point, and DIF was found on multiple relevant background variables. Item 1 'Vision in general' from the "Adjustment" dimension and item 24 'Using tools' from the "Reading and fine work" dimension were removed. Test information was highest for the "Reading and fine work" dimension. Indices for subject separation ranged from 0.83 to 0.94.ConclusionsThe items of the LVQOL showed satisfactory item fit to the graded response model; however, two items were removed because of DIF. The adapted LVQOL with 21 items is DIF-free and therefore seems highly appropriate for use in heterogeneous populations of visually impaired patients.
Highlights
For the Low Vision Quality Of Life questionnaire (LVQOL) it is unknown whether the psychometric properties are satisfactory when an item response theory (IRT) perspective is considered
The detrimental effects of living with vision loss caused by irreversible eye conditions are well reported [1]
De Boer et al reported that the original Low Vision Quality Of Life questionnaire (LVQOL) was one of the best for use in patients with low vision [11,12]; its items are mainly related to difficulties people have in performing certain activities due to their visual disability
Summary
For the Low Vision Quality Of Life questionnaire (LVQOL) it is unknown whether the psychometric properties are satisfactory when an item response theory (IRT) perspective is considered. Research in low vision has primarily focused on older adult populations, because of increased prevalence of agerelated eye conditions in older age [2,3,4,5,6,7,8] Those studies used several vision-related quality-of-life questionnaires which allow to assess the disability suffered in daily life [9,10]. In two subsequent studies on the longitudinal outcomes of low vision rehabilitation, additional comments on the validity of the LVQOL were made using item response theory (IRT); a calibration-process was not performed [5,15] In these studies, which were performed on the data van Nispen et al BMC Medical Research Methodology 2011, 11:125 http://www.biomedcentral.com/1471-2288/11/125 previously used by de Boer et al [13,14], it was concluded that on the dimension “Reading and fine work”, the item invariance assumption did not hold over time. The lack of item invariance might have been a redundant phenomenon if the items had been calibrated in an IRT model beforehand
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