The left hand side of Eq. (1) represents the marginal cost-per-QALY of treating type x,
since the numerator is the marginal cost of abating a statistical illness whilst the denominator
is the number of QALYs gained when a statistical illness is abated. The numerator on the right hand side is the utility value of a prevented illness per unit of health improvement, while the denominator is the marginal utility of income of an additional unit of expenditure spread across society. The right hand side of Eq. (1) is therefore society’s marginal WTP-per-QALY. Therefore, EU maximisation requires that society equalise the marginal cost and marginal WTP-per-QALY for each type.