Health status enters (1) within a utility function that increases in income and decreases
in health status. The restriction that QALYs form a valid cardinal utility function defines
the way in which health enters the utility function (see Johannesson, 1995). Here, the utility
from a given health improvement must not be dependent on the type of person benefiting.
Therefore, the ratio of utility gain and health must be constant, with the utility function
consequently satisfying