Each frontier is a function of one other variable besides health system expenditure. That
is the average years of schooling in the adult population, which is a measure of human capital and therefore of the long-run potential, if not the current or actual, state of development
of the country. It is a proxy for most of the factors outside the health system that
contribute to health status, and probably also to the degree of responsiveness and to how
health is financed. Box 2.5 explains how the upper and lower limits are estimated and how
they should be interpreted.