Rasch analysis of the quality of life and vision function questionnaire.

PURPOSE The Quality of life and Vision Function Questionnaire (QOL-VFQ) was developed using classical test theory to assess outcomes of cataract surgery. The aim of this study was to examine the psychometric properties of this questionnaire using Rasch analysis in a cataract population. METHODS The 17-item QOL-VFQ was self-administered to 389 patients waiting to undergo cataract surgery. The QOL-VFQ and its five subscales were assessed for fit to the Rasch model. Rasch analysis was used to estimate interval level measures of "visual ability" from ordinal scores for the QOL-VFQ and its five subscales. Unidimensionality, item fit, response category performance, and targeting of items to patients were assessed. RESULTS The QOL-VFQ and its five subscales showed ordered category thresholds. Despite removal of two misfitting items the person separation reliability was high and the QOL-VFQ could distinguish among three strata of patient ability. However, there was suboptimal targeting of patient ability to item difficulty as most of the patients had higher levels of visual functioning. None of the subscales demonstrated acceptable psychometric properties. CONCLUSIONS These results support the good overall functioning of the QOL-VFQ in patients with cataract. However, adding more items, that suit the more able patients including those who are awaiting cataract surgery in the fellow eye, will help improve the targeting.