The effect of outliers on confidence interval procedures for cost-effectiveness ratios

Cost-effectiveness ratio (CER) is defined as the ratio of the difference in cost between a test and standard health care programme to the difference i n benefit, respectively. Methods to obtain confidence intervals for CERs ar e either variants of Fieller's method (1954), or bootstrap methods. We stud y the effect of outliers in cost measures on the precision of confidence in terval procedures for CERs. In particular the performance of the procedures under single and multiple case influential deletion diagnostics, respectiv ely, are evaluated. Simulation studies suggest that the bias-corrected perc entile bootstrap procedure gives better precision and coverage under either diagnostic. Copyright (C) 2001 John Wiley & Sons, Ltd.