jmlr jmlr2005 jmlr2005-3 knowledge-graph by maker-knowledge-mining

3 jmlr-2005-A Classification Framework for Anomaly Detection


Source: pdf

Author: Ingo Steinwart, Don Hush, Clint Scovel

Abstract: One way to describe anomalies is by saying that anomalies are not concentrated. This leads to the problem of finding level sets for the data generating density. We interpret this learning problem as a binary classification problem and compare the corresponding classification risk with the standard performance measure for the density level problem. In particular it turns out that the empirical classification risk can serve as an empirical performance measure for the anomaly detection problem. This allows us to compare different anomaly detection algorithms empirically, i.e. with the help of a test set. Furthermore, by the above interpretation we can give a strong justification for the well-known heuristic of artificially sampling “labeled” samples, provided that the sampling plan is well chosen. In particular this enables us to propose a support vector machine (SVM) for anomaly detection for which we can easily establish universal consistency. Finally, we report some experiments which compare our SVM to other commonly used methods including the standard one-class SVM. Keywords: unsupervised learning, anomaly detection, density levels, classification, SVMs

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 GOV Modeling, Algorithms and Informatics Group, CCS-3 Los Alamos National Laboratory Los Alamos, NM 87545, USA Editor: Bernhard Sch¨ lkopf o Abstract One way to describe anomalies is by saying that anomalies are not concentrated. [sent-4, score-0.316]

2 We interpret this learning problem as a binary classification problem and compare the corresponding classification risk with the standard performance measure for the density level problem. [sent-6, score-0.282]

3 In particular it turns out that the empirical classification risk can serve as an empirical performance measure for the anomaly detection problem. [sent-7, score-0.727]

4 This allows us to compare different anomaly detection algorithms empirically, i. [sent-8, score-0.522]

5 In particular this enables us to propose a support vector machine (SVM) for anomaly detection for which we can easily establish universal consistency. [sent-12, score-0.563]

6 Keywords: unsupervised learning, anomaly detection, density levels, classification, SVMs 1. [sent-14, score-0.378]

7 Introduction Anomaly (or novelty) detection aims to detect anomalous observations from a system. [sent-15, score-0.342]

8 This anomaly detection learning problem has many important applications including the detection of e. [sent-18, score-0.769]

9 It is important to note that a typical feature of these applications is that only unlabeled samples are available, and hence one has to make some a-priori assumptions on anomalies in order to be able to distinguish between normal and anomalous future oberservations. [sent-28, score-0.33]

10 One of the most common ways to define anomalies is by saying that anomalies are not concentrated (see e. [sent-29, score-0.316]

11 Obviously, the density level sets {h > ρ}, ρ > 0, describe the concentration of Q. [sent-34, score-0.165]

12 Therefore to define anomalies in terms of the concentration one only has to fix a threshold level ρ > 0 so that a sample x ∈ X is considered to be anomalous whenever h(x) ≤ ρ. [sent-35, score-0.315]

13 We emphasize that given the data-generating distribution Q the choice of µ determines the density h, and consequently anomalies are actually modeled by both µ and ρ. [sent-37, score-0.287]

14 Gaussian mixtures, Parzen windows and k-nearest neighbors density estimates) and therefore for these algorithms defining anomalies is restricted to the choice of ρ. [sent-40, score-0.29]

15 In particular, this is true if we consider a modification of the anomaly detection problem where µ is not known but can be sampled from. [sent-42, score-0.522]

16 Finding level sets of an unknown density is also a well known problem in statistics which has some important applications different from anomaly detection. [sent-44, score-0.408]

17 Unfortunately, the algorithms considered in these articles cannot be used for the anomaly detection problem since the imposed assumptions on h are often tailored to the above applications and are in general unrealistic for anomalies. [sent-50, score-0.522]

18 One of the main problems of anomaly detection—or more precisely density level detection—is the lack of an empirical performance measure which allows us to compare the generalization performance of different algorithms by test samples. [sent-51, score-0.471]

19 By interpreting the density level detection problem as binary classification with respect to an appropriate measure, we show that the corresponding empirical classification risk can serve as such an empirical performance measure for anomaly detection. [sent-52, score-0.897]

20 Furthermore, we compare the excess classification risk with the standard performance measure for the density level detection problem. [sent-53, score-0.608]

21 , 2003) for anomaly detection is to generate a labeled data set by assigning one label to the original unlabeled data and another label to a set of artificially generated data, and then apply a binary classification algorithm. [sent-60, score-0.557]

22 By interpreting the density level detection problem as a binary classification problem we can show that this heuristic can be strongly justified provided that the sampling plan for the artificial samples is chosen in a certain way and the used classification algorithm is well-adopted to this plan. [sent-61, score-0.532]

23 Detecting Density Levels is a Classification Problem We begin with rigorously defining the density level detection (DLD) problem. [sent-66, score-0.38]

24 For the density level detection problem and related tasks this is a common assumption (see e. [sent-73, score-0.38]

25 Furthermore, for a sequence of functions fn : X → R with Sµ,h,ρ ( fn ) → 0 we easily see that sign fn (x) → 1{h>ρ} (x) for µ-almost all x ∈ X, and since Q is absolutely continuous with respect to µ the same convergence holds Q-almost surely. [sent-91, score-0.306]

26 Then the probability measure Q s µ on X ×Y is defined by Q s µ (A) := sEx∼Q 1A (x, 1) + (1 − s)Ex∼µ 1A (x, −1) for all measurable subsets A ⊂ X ×Y . [sent-110, score-0.141]

27 Inspired by this interpretation let us recall that the binary classification risk for a measurable function f : X → R and a distribution P on X ×Y is defined by RP ( f ) = P {(x, y) : sign f (x) = y} , where we define signt := 1 if t > 0 and signt = −1 otherwise. [sent-114, score-0.284]

28 Furthermore, the Bayes risk RP of P is the smallest possible classification risk with respect to P, i. [sent-115, score-0.228]

29 Then for all sequences ( fn ) of measurable functions fn : X → R the following are equivalent: i) SP ( fn ) → 0. [sent-146, score-0.304]

30 Since by Corollary 3 we know µ({h > ρ} 1 {η > 2 }) = 0 it is easy to see that the classification risk of fn can be computed by RP ( fn ) = RP + Z En |2η − 1|dPX . [sent-150, score-0.246]

31 Therefore, the assertion follows from SP ( fn ) = µ(En ). [sent-156, score-0.151]

32 Theorem 4 shows that instead of using SP as a performance measure for the density level detection problem one can alternatively use the classification risk RP (. [sent-157, score-0.529]

33 Then for all measurable f : X → R we have RP ( f ) = 1 1+ρ and define 1 ρ EQ I(1, sign f ) + Eµ I(−1, sign f ) . [sent-163, score-0.234]

34 1+ρ 1+ρ Proof The first assertion directly follows from RP ( f ) = P {(x, y) : sign f (x) = y} = P {(x, 1) : sign f (x) = −1} + P {(x, −1) : sign f (x) = 1} = sQ {sign f = −1} + (1 − s)µ {sign f = 1} = sEQ I(1, sign f ) + (1 − s)Eµ I(−1, sign f ) . [sent-165, score-0.405]

35 As described at the beginning of this section our main goal is to find a performance measure for the density level detection problem which has an empirical counterpart. [sent-168, score-0.443]

36 , xn ) ∈ X n and a measurable function f : X → R we define RT ( f ) := n ρ 1 ∑ I(1, sign f (xi )) + 1 + ρ Eµ I(−1, sign f ) . [sent-174, score-0.234]

37 (1 + ρ)n i=1 If we identify T with the corresponding empirical measure it is easy to see that RT ( f ) is the 1 classification risk with respect to the measure T s µ for s := 1+ρ . [sent-175, score-0.212]

38 ) is strongly connected with another approach for the density level detection problem which is based on the so-called excess mass (see e. [sent-180, score-0.515]

39 To be more precise let us first recall that the excess mass of a measurable function f : X → R is defined by EP ( f ) := Q({ f > 0}) − ρµ({ f > 0}) , where Q, ρ and µ have the usual meaning. [sent-183, score-0.241]

40 By the above proposition we have ET ( f ) = 1 − 1 n ∑ I(1, sign f (xi )) − ρEµ I(−1, sign f ) = 1 − (1 + ρ)RT ( f ) n i=1 (2) for all measurable f : X → R. [sent-193, score-0.287]

41 Now, given a class F of measurable functions from X to R the (empirical) excess mass approach considered e. [sent-194, score-0.241]

42 By equation (2) we see that this approach is actually a type of empirical risk minimization (ERM). [sent-198, score-0.142]

43 956) used (3) for the analysis of a density level detection method which is based on a localized version of the empirical excess mass approach. [sent-213, score-0.543]

44 Mammen and Tsybakov, 1999; Tsybakov, 2004; Steinwart and Scovel, 2004) as the following proposition proved in the appendix shows: Proposition 9 Let µ and Q be distributions on X such that Q has a density h with respect to µ. [sent-216, score-0.156]

45 However it appears that only the “RP ( fn ) → RP ⇒ SP ( fn ) → 0” assertion of Theorem 4 holds instead of equivalence. [sent-245, score-0.217]

46 Therefore we immediately obtain the following corollary Corollary 13 Under the assumptions of Theorem 12 both the DLD-SVM with offset and without ˜ offset are universally consistent with respect to SP (. [sent-322, score-0.153]

47 This gives a strong justification for the well-known heuristic of adding artificial samples to anomaly detection problems with unlabeled data. [sent-328, score-0.625]

48 Experiments We present experimental results for anomaly detection problems where the set X is a subset of Rd . [sent-331, score-0.522]

49 Based on Definition 6 we define the empirical risk of f with respect to (S, S ) to be R(S,S ) ( f ) = ρ 1 ∑ I(1, sign f (x)) + (1 + ρ)|S | ∑ I(−1, sign f (x)). [sent-335, score-0.27]

50 the rate at which samples will be labeled anomalous by f ), and the quantity 1 |S | ∑x∈S I(−1, sign f (x)) is an estimate of µ({ f > 0}) which we call the volume of the predicted normal set. [sent-340, score-0.201]

51 Also, from the expression for the risk in Proposition 5 it is clear that for any two functions with the same alarm rate we prefer the function with the smaller volume and vice versa. [sent-344, score-0.18]

52 We consider three different anomaly detection problems, two are synthetic and one is an application in cybersecurity. [sent-347, score-0.522]

53 In each case we define a problem instance to be a triplet consisting of samples from Q, samples from µ, and a value for the density level ρ. [sent-348, score-0.217]

54 (2001) and the o others (including the Parzen windows method) are based on some of the most common parametric and non-parametric statistical methods for density-based anomaly detection in Rd . [sent-351, score-0.551]

55 The core procedures are run on the training sets and the values of the free parameters are chosen to minimize the empirical risk (8) on the validation sets. [sent-358, score-0.169]

56 The regularization parameters λ and σ2 are chosen to (approximately) minimize the empirical risk R(V,V ) ( f ) on the validation sets. [sent-363, score-0.169]

57 In particular, for each value of λ from a fixed grid we seek a minimizer over σ2 by evaluating the validation risk at a coarse grid of σ2 values and then performing a Golden search over the interval defined by the two σ2 values on either side of the coarse grid minimum. [sent-365, score-0.309]

58 2 As the overall search proceeds the (λ, σ2 ) pair with the smallest validation risk is retained. [sent-366, score-0.141]

59 In particular both ν and σ2 are chosen to (approximately) minimize the validation risk using the search procedure described above for the DLD–SVM where the grid search for λ is replaced by a Golden search (over [0, 1]) for ν. [sent-376, score-0.197]

60 The GML algorithm produces a function f = g − t where t is an offset and g is a Gaussian probability density function whose mean and inverse covariance are determined from maximum likelihood estimates formed from the training data T (see e. [sent-377, score-0.166]

61 Once the parameters of g are determined the offset t is chosen to minimize the training risk R(T,T ) . [sent-382, score-0.177]

62 The regularization parameter λ is chosen to (approximately) minimize the validation risk by searching a fixed grid of λ values. [sent-383, score-0.197]

63 222 A C LASSIFICATION F RAMEWORK FOR A NOMALY D ETECTION Number of Q samples Number of µ samples λ grid (DLD–SVM/GML/MGML) σ2 grid (DLD–SVM/1CLASS–SVM) Train 1000 2000 Validate 500 2000 Test 100,000 100,000 1. [sent-388, score-0.196]

64 for all inverse covariance estimates and both λ and K are chosen to (approximately) minimize the validation risk by searching a fixed grid of (λ, K) values. [sent-408, score-0.197]

65 Figures 1(a) and 1(c) plot the empirical risk R(W,W ) versus ρ while Figures 1(b) and 1(d) plot the corresponding performance curves. [sent-427, score-0.142]

66 These results are significant because values of ρ substantially larger than one appear to have little utility here since they yield alarm rates that do not conform to our notion that anomalies are rare events. [sent-434, score-0.224]

67 In addition ρ 1 appears to have little utility in the general anomaly detection problem since it defines anomalies in regions where the concentration of Q is much larger than the concentration of µ, which is contrary to our premise that anomalies are not concentrated. [sent-435, score-0.902]

68 005 to 50 and the results are summarized by the empirical risk curve in Figure 2(a) and the corresponding performance curve in Figure 2(b). [sent-490, score-0.142]

69 The empirical risk values for DLD–SVM and MGML are nearly identical except for ρ = 0. [sent-491, score-0.142]

70 Number of Q samples Number of µ samples λ grid (DLD–SVM/GML/MGML) σ2 grid (DLD–SVM/1CLASS–SVM) Train 4000 10,000 Validate 2000 100,000 Test 5664 100,000 0. [sent-493, score-0.196]

71 Except for this case the empirical risk values for DLD–SVM and MGML are much better than 1CLASS–SVM and GML at nearly all values of ρ. [sent-514, score-0.142]

72 The performance curves confirm the superiority of DLD–SVM and MGML, but also reveal differences not easily seen in the empirical risk curves. [sent-515, score-0.169]

73 For example, all four methods produced some solutions with identical performance estimates for different values of ρ which is reflected by the fact that the performance curves show fewer points than the corresponding empirical risk curves. [sent-516, score-0.169]

74 Discussion A review of the literature on anomaly detection suggests that there are many ways to characterize anomalies (see e. [sent-533, score-0.68]

75 In this work we assumed that anomalies are not concentrated. [sent-536, score-0.158]

76 This assumption can be specified by choosing a reference measure µ which determines a density and a level value ρ. [sent-537, score-0.194]

77 The density then quantifies the degree of concentration and the density level ρ establishes a threshold on the degree that determines anomalies. [sent-538, score-0.294]

78 In practice the user chooses µ and ρ to capture some notion of anomaly that he deems relevant to the application. [sent-540, score-0.275]

79 This paper advances the existing state of “density based” anomaly detection in the following ways. [sent-541, score-0.522]

80 Therefore we accommodate a larger class of anomaly detection problems. [sent-543, score-0.522]

81 Consequently, it has been difficult to compare different methods for anomaly detection in practice. [sent-549, score-0.522]

82 We have demonstrated this approach which is a rigorous variant of a well-known heuristic for anomaly detection in the formulation of the DLD-SVM. [sent-554, score-0.548]

83 These advances have created a situation in which much of the knowledge on classification can now be used for anomaly detection. [sent-555, score-0.275]

84 Consequently, we expect substantial advances in anomaly detection in the future. [sent-556, score-0.522]

85 Finally let us consider a different learning scenario in which anomaly detection methods are also commonly employed. [sent-557, score-0.522]

86 Hidden classification problems for example occur in network intrusion detection problems where it is impractical to obtain labels. [sent-561, score-0.275]

87 This means for example that when an anomaly detection method is used to produce a classifier f for a HCP its anomaly detection performance RP ( f ) with 1 P := Q s µ and s := 1+ρ may be very different from its hidden classification performance Rν ( f ). [sent-573, score-1.044]

88 Another consequence of the above considerations is that the common practice of measuring the performance of anomaly detection algorithms on (hidden) binary classification problems is problematic. [sent-579, score-0.522]

89 Indeed, the obtained classification errors depend on the model error and thus they provide an inadequate description how well the algorithms solve the anomaly detection problem. [sent-580, score-0.522]

90 In conclusion although there are clear similiarities between the use of the DLD formalism for anomaly detection and its use for the HCP there is also an important difference. [sent-584, score-0.522]

91 In the first case the specification of µ and ρ determines the definition of anomalies and therefore there is no model error, whereas in the second case the model error is determined by the choice of µ and ρ. [sent-585, score-0.184]

92 2) gives a sufficient condition for the existence of a regular conditional probability: 228 A C LASSIFICATION F RAMEWORK FOR A NOMALY D ETECTION Theorem 17 If Y is a Polish space then a regular conditional probability P( . [sent-614, score-0.178]

93 Therefore we find 1+t 1−t ρ≤h≤ ρ 1+t 1−t |2η − 1| ≤ t ≤ PX (1 − tr )ρ ≤ h ≤ (1 + tr )ρ = PX (1 − tl )ρ ≤ h ≤ (1 + tr )ρ ⊂ Hence for all sufficiently small t > 0 with t < = |h − ρ| ≤ tr ρ . [sent-644, score-0.182]

94 Applications of probability density estimation to the detection of abnormal conditions in engineering. [sent-704, score-0.35]

95 Using artificial anomalies to detect unknown and known network intrusions. [sent-728, score-0.158]

96 Support vector novelty detection applied to o jet engine vibration spectra. [sent-755, score-0.345]

97 The use of novelty detection techniques for monitoring high-integrity plant. [sent-773, score-0.307]

98 Network intrusion and fault detection: a statistical anomaly approach. [sent-786, score-0.303]

99 Measuring mass concentrations and estimating density contour clusters—an excess mass aproach. [sent-819, score-0.294]

100 Novelty detection for the identification of masses in mammograms. [sent-882, score-0.247]


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