A critical re-evaluation of the regression model specification in the US D1 EQ-5D value function

The dominant procedure for defining values for EQ-5D health states involves regression modeling. These regression models have typically included a constant term, interpreted as the utility loss associated with any movement away from perfect health. The authors of the United States EQ-5D valuation study replaced this constant with a variable, D1, which corresponds to the number of impaired dimensions beyond the first. The aim of this study was to illustrate how the use of the D1 variable in place of a constant is problematic.We compared the original D1 regression model with a mathematically equivalent model with a constant term. Comparisons included implications for the magnitude and statistical significance of the coefficients, multicollinearity (variance inflation factors, or VIFs), number of calculation steps needed to determine tariff values, and consequences for tariff interpretation.Using the D1 variable in place of a constant shifted all dummy variable coefficients away from zero by the value of the constant, greatly increased the multicollinearity of the model (maximum VIF of 113.2 vs. 21.2), and increased the mean number of calculation steps required to determine health state values.Using the D1 variable in place of a constant constitutes an unnecessary complication of the model, obscures the fact that at least two of the main effect dummy variables are statistically nonsignificant, and complicates and biases interpretation of the tariff algorithm.The EQ-5D, a generic instrument for measuring health-related quality of life (HRQoL), is used extensively in cost-utility/cost-effectiveness analyses [1,2]. The EQ-5D measures health along five dimensions (mobility, self-care, usual activities, pain/discomfort, and anxiety/depression). Each of these dimensions can be described at three levels of functioning, corresponding to (1) no problems, (2) some problems, and (3) extreme problems. This gives a total of 243 possible combinations, or health states. Specific heal