title: Cost-effectiveness acceptability curves--facts, fallacies and frequently asked questions.
abstract: 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 (lambda). 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 lambda; 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.
authors.affiliation: Centre for Evaluation of Medicines, St Joseph's Hospital, Hamilton, Canada. ealf100@york.ac.uk
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Fenwick
authors.forename: Bernie J
authors.initials: BJ
authors.lastname: O'Brien
authors.forename: Andrew
authors.initials: A
authors.lastname: Briggs
mesh_terms: D001499:Bayes Theorem; D003362:Cost-Benefit Analysis / Q000379:methods* / Q000706:statistics & numerical data; D006801:Humans; D019057:Quality-Adjusted Life Years; D014481:United States