Estimated smooth functions and showing the nonlinear effects of age and body mass index (BMI) on the mean EQ-5D index score. The solid curves represent GAM estimates using a thin plate regression spline function with estimated 6.2 and 4.7 effective degrees of freedom for age and BMI, respectively. The shaded areas represent approximate 95% pointwise confidence intervals. The functions are fixed around the mean value of the EQ-5D index score. Due to estimation uncertainty for outliers, values for seven subjects over 90 years and one subject with a BMI >50 are not displayed.