Cost-effectiveness acceptability curves and a reluctance to lose.

Cost-effectiveness acceptability curves (CEACs) are a method used to present uncertainty surrounding incremental cost-effectiveness ratios (ICERs). Construction of the curves relies on the assumption that the willingness to pay (WTP) for health gain is identical to the willingness to accept (WTA) health loss. The objective of this paper is to explore the impact that differences between WTP and WTA health changes have on CEACs. Previous empirical evidence has shown that the relationship between WTP and WTA is not 1:1. The discrepancy between WTP and WTA for health changes can be expressed as a ratio: the accept/reject ratio (which can vary between 1 and infinity). Depending on this ratio, the area within the southwest quadrant of the cost-effectiveness plane in which any bootstrap cost-effect pairs will be considered to be cost effective will be smaller, resulting in a lower CEAC. We used data from two clinical trials to illustrate that relaxing the 1:1 WTP/WTA assumption has an impact on the CEACs. Given the difficulty in assessing the accept/reject ratio for every evaluation, we suggest presenting a series of CEACs for a range of values for the accept/reject ratio, including 1 and infinite. Although it is not possible to explain this phenomenon within the extra-welfarist framework, it has been shown empirically that individuals give a higher valuation to the removal of effective therapies than to the introduction of new therapies that are more costly and effective. In cost-effectiveness analyses where uncertainty of the ICER covers the southwest quadrant of the cost-effectiveness plane, the discrepancy between societies' WTP and WTA should be indicated by drawing multiple CEACs.