where income is constant, the societal WTP-per-QALY becomes constant with respect
to health status. A representation of the type of u_ consistent with this assumption is
displayed in Fig. 1. Note that we require the ‘constant incomes’ assumption since a cursory
examination of the diagram reveals that the utility gain from a prevented illness is a function
of income. Contrast this with Fig. 2, which shows a u_ function where the WTP-per-QALY
assumption is violated. At any positive level of income, the lines (though equidistant in
health status) exhibit decreasing marginal gains in utility.