#### Title

EMPIRICAL PROCESSES AND ROC CURVES WITH AN APPLICATION TO LINEAR COMBINATIONS OF DIAGNOSTIC TESTS

#### Year of Publication

2008

#### Degree Name

Doctor of Philosophy (PhD)

#### Document Type

Dissertation

#### College

Arts and Sciences

#### Department

Statistics

#### First Advisor

Dr. Constance L. Wood

#### Second Advisor

Dr. Arne C. Bathke

#### Abstract

The Receiver Operating Characteristic (ROC) curve is the plot of Sensitivity vs. 1- Specificity of a quantitative diagnostic test, for a wide range of cut-off points *c*. The empirical ROC curve is probably the most used nonparametric estimator of the ROC curve. The asymptotic properties of this estimator were first developed by Hsieh and Turnbull (1996) based on strong approximations for quantile processes. Jensen *et al*. (2000) provided a general method to obtain regional confidence bands for the empirical ROC curve, based on its asymptotic distribution.

Since most biomarkers do not have high enough sensitivity and specificity to qualify for good diagnostic test, a combination of biomarkers may result in a better diagnostic test than each one taken alone. Su and Liu (1993) proved that, if the panel of biomarkers is multivariate normally distributed for both diseased and non-diseased populations, then the linear combination, using Fisher's linear discriminant coefficients, maximizes the area under the ROC curve of the newly formed diagnostic test, called the generalized ROC curve. In this dissertation, we will derive the asymptotic properties of the generalized empirical ROC curve, the nonparametric estimator of the generalized ROC curve, by using the empirical processes theory as in van der Vaart (1998). The pivotal result used in finding the asymptotic behavior of the proposed nonparametric is the result on random functions which incorporate estimators as developed by van der Vaart (1998). By using this powerful lemma we will be able to decompose an equivalent process into a sum of two other processes, usually called the brownian bridge and the drift term, via Donsker classes of functions. Using a uniform convergence rate result given by Pollard (1984), we derive the limiting process of the drift term. Due to the independence of the random samples, the asymptotic distribution of the generalized empirical ROC process will be the sum of the asymptotic distributions of the decomposed processes. For completeness, we will first re-derive the asymptotic properties of the empirical ROC curve in the univariate case, using the same technique described before. The methodology is used to combine biomarkers in order to discriminate lung cancer patients from normals.

#### Recommended Citation

Chirila, Costel, "EMPIRICAL PROCESSES AND ROC CURVES WITH AN APPLICATION TO LINEAR COMBINATIONS OF DIAGNOSTIC TESTS" (2008). *University of Kentucky Doctoral Dissertations*. 674.

http://uknowledge.uky.edu/gradschool_diss/674