Content-based interpretation aids for health-related quality of life measures in clinical practice. An example for the visual function index (VF-14)
10.1023/B:QURE.0000015298.09085.b0 Cite this article as: Valderas, J., Alonso, J., Prieto, L. et al. Qual Life Res (2004) 13: 35. doi:10.1023/B:QURE.0000015298.09085.b0 Abstract Background: In spite of a well-established development of instruments, difficulty in interpreting health related quality of life scores may limit its use in clinical practice. Objective: To develop generalizable interpretation aids for a measure of perceived functional visual status, the VF-14 index. Design: Item Response Theory (Rasch analysis) was used to analyze the performance of VF-14 items. The ‘ruler’ aid was derived from the most difficult activity (item) a patient is able to do without difficulty; the ‘clinical scenarios’ aid, first identified all significantly different clusters of items within the index and then estimated the mean expected difficulty (responses) to perform a benchmark item in each cluster. Setting: The study was conducted in four hospitals and six ambulatory cataract surgery centers in Barcelona, Spain. Patients: One hundred and ninety-eight patients scheduled for first eye cataracts surgery. Measurements: The self-reported VF-14 index and clinical measures were used. Results: All VF-14 items were found unidimensional with three items showing only partial misfit. For a patient with a VF-14 Rasch score of 71, the ‘ruler’ aid indicated that ‘ doing fine handwork’ would be the most requiring activity he/she would perform without difficulty. The ‘clinical scenarios’ aid estimated that such a patient would be unable to ‘ drive at night’, would have some difficulty ‘ reading small print’ and no difficulty ‘ doing fine handwork’, ‘ watching TV’ or ‘ recognizing people’. Concordance between modeled and observed responses was fair to substantial. Conclusions: Simple content-based interpretation aids for the VF-14 scores were developed that should facilitate its use in clinical practice. These aids should be easily generalizable to other quality of life instruments. Clinical interpretability Functional status Health-related quality of life Item response theory Psychometric methods Questionnaires Validity References
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