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Questionnaire offers valid measure of visual disability despite poor targeting
Optom Vis Sci. 2009;86(10):1160-1168.
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The Visual Function questionnaire provides a more valid statistical model of visual disability in cataract patients than the Quality of Life questionnaire, according to a study.
"The [Quality of Life] questionnaire is not a valid measure of [quality of life]," the study authors said. "However, the [Visual Function] questionnaire is a reliable and valid measure of visual disability in patients with cataract, and although targeting was suboptimal in a developed country, it may be optimal in a developing country as was originally intended."
Investigators mailed the Visual Function (VF) and Quality of Life (QOL) questionnaires to 489 cataract patients in Australia. They undertook Rasch analysis to evaluate statistical properties such as unidimensionality, discrimination, targeting and differential item functioning.
The analysis demonstrated that the VF questionnaire showed good metric properties and was free of significant differential item functioning. The QOL questionnaire showed insufficient person separation, ill-fitting items and appreciable multidimensionality. The presence of a self-care subscale was suggested but the subscale was invalid because of poor person separation.
"Deleting these contrasting items from the QOL questionnaire removed multidimensionality and item misfit but did not improve person separation, indicating that it could not effectively discriminate between the participant's QOL," the authors said.
Both questionnaires showed poor targeting, an indication that items were too easy and that subjects had a higher quality of life than allowed by the items.
Three items in the QOL questionnaire showed marked differential item functioning according to age and the presence of systemic comorbidities. None of the subscales functioned properly, the authors said.
This paper demonstrates the power of Rasch analysis to evaluate the measurement properties of visual function questionnaires (VFQs) and other self-report measures. Although 50 years old, Rasch analysis is relatively new to clinical vision research, and most existing VFQs were developed without the benefit of this rigorous and quantitative analytic tool. The important take-home message of the Gothwal et al paper, and others similar to it, is that many of the existing VFQs have shortcomings as measurement tools and are not well-targeted to the clinical populations for which they are intended. The work of Gothwal and her colleagues goes even further to raise the question of whether or not it is possible to measure functional or quality of life changes in response to cataract surgery when the patient has a well-seeing fellow eye. Also see Lansingh VC, Carter MJ Arch Ophthalmol. 2009;127(9):1183 and Frick KD, Massof RW op cit., 1205.
– Robert W. Massof, MD
Professor of
Ophthalmology and Neuroscience, Wilmer Eye Institute, Johns Hopkins University
School of Medicine, Baltimore