Abstract
BackgroundThe EQ-5D is a generic health-related quality of life instrument (five dimensions with three levels, 243 health states), used extensively in cost-utility/cost-effectiveness analyses. EQ-5D health states are assigned values on a scale anchored in perfect health (1) and death (0).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.MethodsWe 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.ResultsUsing 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.DiscussionUsing 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.
Highlights
The EQ-5D is a generic health-related quality of life instrument, used extensively in cost-utility/cost-effectiveness analyses
We have focused on the problems related to the way the authors of the US valuation study replaced the constant term from previous regression models with the D1 variable
The US EQ-5D tariff would be better represented using the D1c notification presented in this paper
Summary
The EQ-5D is a generic health-related quality of life instrument (five dimensions with three levels, 243 health states), used extensively in cost-utility/cost-effectiveness analyses. 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 EQ-5D measures health along five dimensions (mobility, selfcare, 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. Since directly valuing all EQ-5D health states was considered impractical, EQ5D valuation studies have typically elicited values for a subset of the EQ-5D health states, and values for the full set have been assigned using regression modeling, making the regression procedure and model crucial components of the EQ-5D system [5]
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