Cost-effectiveness acceptability curves - facts, fallacies and frequently asked questions

Cost‐effectiveness acceptability curves (CEACs) have been widely adopted as a method to quantify and graphically represent uncertainty in economic evaluation studies of health‐care technologies. However, there remain some common fallacies regarding the nature and shape of CEACs that largely result from the ‘textbook’ illustration of the CEAC. This ‘textbook’ CEAC shows a smooth curve starting at probability 0, with an asymptote to 1 for higher money values of the health outcome (λ). But this familiar ‘ogive’ shape which makes the ‘textbook’ CEAC look like a cumulative distribution function is just one special case of the CEAC. The reality is that the CEAC can take many shapes and turns because it is a graphic transformation from the cost‐effectiveness plane, where the joint density of incremental costs and effects may ‘straddle’ quadrants with attendant discontinuities and asymptotes. In fact CEACs: (i) do not have to cut the y‐axis at 0; (ii) do not have to asymptote to 1; (iii) are not always monotonically increasing in λ; and (iv) do not represent cumulative distribution functions (cdfs). Within this paper we present a ‘gallery’ of CEACs in order to identify the fallacies and illustrate the facts surrounding the CEAC. The aim of the paper is to serve as a reference tool to accompany the increased use of CEACs within major medical journals. Copyright © 2004 John Wiley & Sons, Ltd.