Validation of a visual function and quality of life instrument in an urban Indian population with uncorrected refractive error using Rasch analysis

To develop and validate a patient-reported outcome measure of refractive error for persons living in developing countries.A 21-item visual function questionnaire was developed from existing instruments through pilot testing in 51 urban Indian adults with refractive error. This was then administered to 400 adults aged 18 to 45 in Delhi, India. Rasch analysis was performed to determine item fit, differential item functioning (DIF), targeting of the instrument to the study population, and instrument precision.The removal of 5 items from the instrument resulted in better fit to the Rasch model. Reducing the number of possible responses from five to four further improved fit. Analysis of the person-item map including the Fisher information function demonstrated acceptable targeting of the instrument to this population. The person separation index was good (2.03), demonstrating ability to differentiate between 3 strata of respondents.Revisions to existing instruments to measure visual function in persons with refractive error resulted in an instrument that conformed to the Rasch model. Identifying activities that are more difficult to perform with refractive error in this setting might further improve the instrument. The 16-item instrument was able to differentiate among levels of impairment due to uncorrected refractive error and could be used to assess refractive programs in similar settings.