For a Bi-Exponentiated Weibull model, the authors obtain a general AUC formula and derive the maximum likelihood estimator of AUC and its asymptotic property. A simulation study is carried out to illustrate the finite sample size performance.
ROC Curve and AUC for a Bi-Exponentiated Weibull ModelIn medical science, a diagnostic test result called a biomarker [1,2] is an indicator for disease status of patients. The accuracy of a medical diagnostic test is typically evaluated by sensitivity and specificity. Receiver Operating Characteristic (ROC) curve is a graphical representation of the relationship between sensitivity and specificity. The area under the ROC curve (AUC) is an overall performance measure for the biomarker. Hence the main issue in assessing the accuracy of a diagnostic test is to estimate the ROC curve and its AUC. Suppose that there are two groups of study subjects: diseased and nondiseased. Let be a continuous biomarker. Assume that the larger is, the more likely a subject is diseased. That is, a subject is classified as positive or diseased status if > and as negative or nondiseased status otherwise, where is a cutoff point. Let be the disease status:= 1 represents diseased population while = 0 represents nondiseased population. Sensitivity of is defined as the probability of being correctly classified as disease status and specificity as the probability of being correctly classified as nondisease status. That is, sensitivity ( ) = ( > | = 1) , specificity ( ) = ( < | = 0) .(1) Let = { | = 1} and = { | = 0} be the biomarkers for diseased and nondiseased subjects with survival functions 1 and 0 , respectively. Then sensitivity ( ) = 1 ( ) ,The ROC function (curve) is defined asNotice that ROC curve is a monotonic increasing curve in a unit square starting at (0, 0) and ending at (1, 1). A good biomarker has a very concave down ROC curve. We say a bivariate random vector ( , ) is a Bi-model if and are independent and are from the same distribution family but with different parameters. Pepe [3] summarizes ROC analysis and notices that among Bi-model, Bi-normal model has been the most popular one. The ROC curve method traces back to Green and Swets [4] in the signal detection theory. Bi-normal model assumes the existence of a monotonic increasing transformation such that the biomarker can be transformed into normally distributed for both diseased and nondiseased patients. Pepe [3, Results 4.1 and 4.4] shows that the ROC curve of a biomarker is invariant under a monotone increasing transformation. Hence, a good estimator of ROC curve should satisfy this invariance property.