Re-evaluating a vision-related quality of life questionnaire with item response theory (IRT) and differential item functioning (DIF) analyses

BMC Medical Research Methodology

Ger HMB van Rens⁴  Maaike Langelaan¹  Dirk L Knol²  Ruth MA van Nispen³ 

[1] Netherlands Institute for Health Services Research (NIVEL), Utrecht, the Netherlands;Department of Epidemiology and Biostatistics, VU University Medical Center, Amsterdam, the Netherlands;EMGO+ Institute for Health and Care Research, VU University Medical Center, Amsterdam, the Netherlands;Department of Ophthalmology, Elkerliek Hospital, Helmond, the Netherlands

关键词: Differential item functioning;    Graded response model;    Item response theory;    Vision-related quality of life;    Visual impairment;   

Others  :  1139993
DOI  :  10.1186/1471-2288-11-125

 received in 2010-09-16, accepted in 2011-09-02,  发布年份 2011

【 摘 要 】

Background

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. This study evaluates some essential psychometric properties of the LVQOL questionnaire in an IRT model, and investigates differential item functioning (DIF).

Methods

Cross-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.

Results

All 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.

Conclusions

The 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.

【 授权许可】

   
2011 van Nispen et al; licensee BioMed Central Ltd.

【 图 表 】

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