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Title:Universal outlier hypothesis testing with applications to anomaly detection
Author(s):Li, Yun
Director of Research:Veeravalli, Venugopal
Doctoral Committee Chair(s):Veeravalli, Venugopal
Doctoral Committee Member(s):Moulin, Pierre; Mehta, Prashant; Varshney, Lav
Department / Program:Electrical & Computer Engineering
Discipline:Electrical & Computer Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):universal outlier hypothesis testing
anomaly detection
generalized likelihood test
multihypothesis sequential probability ratio test
cluster analysis
Abstract:Outlier hypothesis testing is studied in a universal setting. Multiple sequences of observations are collected, a small subset (possibly empty) of which are outliers. A sequence is considered an outlier if the observations in that sequence are distributed according to an “outlier” distribution, distinct from the “typical” distribution governing the observations in the majority of the sequences. The outlier and typical distributions are not fully known, and they can be arbitrarily close. The goal is to design a universal test to best discern the outlier sequence(s). Both fixed sample size and sequential settings are considered in this dissertation. In the fixed sample size setting, for models with exactly one outlier, the generalized likelihood test is shown to be universally exponentially consistent. A single letter characterization of the error exponent achieved by such a test is derived, and it is shown that the test achieves the optimal error exponent asymptotically as the number of sequences goes to infinity. When the null hypothesis with no outlier is included, a modification of the generalized likelihood test is shown to achieve the same error exponent under each non-null hypothesis, and also consistency under the null hypothesis. Then, models with multiple outliers are considered. When the outliers can be distinctly distributed, in order to achieve exponential consistency, it is shown that it is essential that the number of outliers be known at the outset. For the setting with a known number of distinctly distributed outliers, the generalized likelihood test is shown to be universally exponentially consistent. The limiting error exponent achieved by such a test is characterized, and the test is shown to be asymptotically exponentially consistent. For the setting with an unknown number of identically distributed outliers, a modification of the generalized likelihood test is shown to achieve a positive error exponent under each non-null hypothesis, and consistency under the null hypothesis. In the sequential setting, a test with the flavor of the repeated significance test is proposed. The test is shown to be universally consistent, and universally exponentially consistent under non-null hypotheses. In addition, with the typical distribution being known, the test is shown to be asymptotically optimal universally when the number of outliers is the largest possible. In all cases, the asymptotic performance of the proposed test when none of the underlying distributions is known is shown to converge to that when only the typical distribution is known as the number of sequences goes to infinity. For models with continuous alphabets, a test with the same structure as the generalized likelihood test is proposed, and it is shown to be universally consistent. It is also demonstrated that there is a close connection between universal outlier hypothesis testing and cluster analysis. The performance of various proposed tests is evaluated against a synthetic data set, and contrasted with that of two popular clustering methods. Applied to a real data set for spam detection, the sequential test is shown to outperform the fixed sample size test when the lengths of the sequences exceed a certain value. In addition, the performance of the proposed tests is shown to be superior to that of another kernel-based test for large sample sizes.
Issue Date:2015-10-07
Type:Thesis
URI:http://hdl.handle.net/2142/88955
Rights Information:Copyright 2015 Yun Li
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12


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