Objective: To compare three separate methods for establishing interpretability for a health status measure, the Hydrocephalus Outcome Questionnaire (HOQ).
Study design and setting: The mothers of children with hydrocephalus attending the outpatient clinics at a pediatric hospital completed the HOQ (for which scores can range from 0 to 1.0 with the smallest possible incremental change being .005), the Health Utilities Index-2 (HUI-2), and a global rating of their child's health. The surgeon for the child also provided a global rating of the child's health following their visit. These data were used to calculate (i) the minimal important difference (MID) based on global health ratings, (ii) the MID based on an effect size approach, and (iii) the conversion of numerical HOQ scores into health utility scores obtained from the HUI-2.
Results: Based on mothers' responses (n = 79) and surgeons' responses (n = 61), respectively, the MID for the HOQ was estimated to be .12 and .10. Using the effect size approach, the MID was estimated to be much lower at .03. HOQ scores were found to be readily translatable to HUI-2 utility scores using a simple linear transformation. The mean utility score for this sample of patients was .77.
Conclusions: Two methods for determining the MID yielded quantitatively different results. Conversion of numerical health status scores to utility scores was done successfully and providing another element of interpretability.