Enhanced Interpretation of Instrument Scales Using the Rasch Model

Abstract

Objectives: To enhance the interpretability of instrument scales in general and the National Eye Institute Visual Function Questionnaire in particular. Methods: The Rasch model was applied to the German version of the 39-item National Eye Institute Visual Function Questionnaire in patients with age-related macular degeneration. Two important sub-scales, near vision and distance vision, were used in this work. The likelihood of performing a task with little or no difficulty was selected as an indicator of a patient’s quality of life. Each possible subscale score was related to the likelihood of performing each task constituting the subscale with little or no difficulty. Results: The Rasch rating scale model was found to fit both subscales based on acceptable infit mean squares, a wide range of item difficulties, and monotonically increasing thresholds. Rasch category characteristic curves for each item in the subscale were back-transformed to the original scale of measurement and placed on a single graph. The graph provides a depiction of the relative difficulty of tasks, the probability of an individual performing a task without difficulty, and the expected mean performance of a group of individuals based on a subscale score. Results could be extended to access the impact of an intervention on the visual quality of life. Conclusions: This approach provides enhanced meaning to a subscale score while permitting the quantification of the improvement in the quality of life resulting from an intervention.