The notion of optimal cost-efficiency. The horizontal axis illustrates the total cost associated with an exposure measurement strategy, and the vertical axis shows the variance of the resulting mean exposure. The frontier curve illustrates the minimal obtainable variance at each level of spending, i.e. the best possible statistical performance, e.g. s2
μ*, at a particular total cost, e.g. c*. Strategies above the frontier are, in principle, possible, but do not yield an optimal performance. No strategies occur below the frontier.