nips nips2013 nips2013-232 knowledge-graph by maker-knowledge-mining

232 nips-2013-Online PCA for Contaminated Data


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Author: Jiashi Feng, Huan Xu, Shie Mannor, Shuicheng Yan

Abstract: We consider the online Principal Component Analysis (PCA) where contaminated samples (containing outliers) are revealed sequentially to the Principal Components (PCs) estimator. Due to their sensitiveness to outliers, previous online PCA algorithms fail in this case and their results can be arbitrarily skewed by the outliers. Here we propose the online robust PCA algorithm, which is able to improve the PCs estimation upon an initial one steadily, even when faced with a constant fraction of outliers. We show that the final result of the proposed online RPCA has an acceptable degradation from the optimum. Actually, under mild conditions, online RPCA achieves the maximal robustness with a 50% breakdown point. Moreover, online RPCA is shown to be efficient for both storage and computation, since it need not re-explore the previous samples as in traditional robust PCA algorithms. This endows online RPCA with scalability for large scale data. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 sg Abstract We consider the online Principal Component Analysis (PCA) where contaminated samples (containing outliers) are revealed sequentially to the Principal Components (PCs) estimator. [sent-10, score-0.437]

2 Due to their sensitiveness to outliers, previous online PCA algorithms fail in this case and their results can be arbitrarily skewed by the outliers. [sent-11, score-0.287]

3 Here we propose the online robust PCA algorithm, which is able to improve the PCs estimation upon an initial one steadily, even when faced with a constant fraction of outliers. [sent-12, score-0.471]

4 We show that the final result of the proposed online RPCA has an acceptable degradation from the optimum. [sent-13, score-0.254]

5 Actually, under mild conditions, online RPCA achieves the maximal robustness with a 50% breakdown point. [sent-14, score-0.342]

6 Moreover, online RPCA is shown to be efficient for both storage and computation, since it need not re-explore the previous samples as in traditional robust PCA algorithms. [sent-15, score-0.426]

7 This endows online RPCA with scalability for large scale data. [sent-16, score-0.264]

8 1 Introduction In this paper, we investigate the problem of robust Principal Component Analysis (PCA) in an online fashion. [sent-17, score-0.348]

9 PCA aims to construct a low-dimensional subspace based on a set of principal components (PCs) to approximate all the observed samples in the least-square sense [19]. [sent-18, score-0.234]

10 To address this problem, several online PCA algorithms have been developed in literature [15, 23, 10]. [sent-20, score-0.237]

11 For online PCA, at each time instance, a new sample is revealed, and the PCs estimation is updated accordingly without having to re-explore all previous samples. [sent-21, score-0.294]

12 Significant advantages of online PCA algorithms include independence of their storage space requirement of the number of samples, and handling newly revealed samples quite efficiently. [sent-22, score-0.423]

13 However, all of these methods work in batch mode and cannot handle sequentially revealed samples in the online learning framework. [sent-25, score-0.529]

14 1 In this work, we propose a novel online Robust PCA algorithm to handle contaminated sample set, i. [sent-30, score-0.301]

15 , sample set that comprises both authentic samples (non-corrupted samples) and outliers (corrupted samples), which are revealed sequentially to the algorithm. [sent-32, score-0.671]

16 Previous online PCA algorithms generally fail in this case, since they update the PCs estimation through minimizing the quadratic error w. [sent-33, score-0.289]

17 The outliers may manipulate the PCs estimation severely and the result can be arbitrarily bad. [sent-37, score-0.33]

18 In contrast, the proposed online RPCA is shown to be robust to the outliers. [sent-38, score-0.365]

19 This is different from previous online PCA methods, where each and every new sample is admitted. [sent-40, score-0.258]

20 The probabilistic admittion/rejection procedure endows online RPCA with the ability to reject more outliers than authentic samples and thus alleviates the affect of outliers and robustifies the PCs estimation. [sent-41, score-1.111]

21 Indeed, we show that given a proper initial estimation, online RPCA is able to steadily improve its output until convergence. [sent-42, score-0.319]

22 In fact, under mild conditions, online RPCA can be resistent to 50% outliers, namely having a 50% breakdown point. [sent-44, score-0.335]

23 Compared with previous robust PCA methods (typically works in batch mode), online RPCA only needs to maintain a covariance matrix whose size is independent of the number of data points. [sent-46, score-0.483]

24 Upon accepting a newly revealed sample, online RPCA updates the PCs estimation accordingly without re-exploring the previous samples. [sent-47, score-0.405]

25 Thus, online RPCA can deal with large amounts of data with low storage expense. [sent-48, score-0.258]

26 To the best of our knowledge, this is the first attempt to make online PCA work for outlier-corrupted data, with theoretical performance guarantees. [sent-50, score-0.237]

27 To address this issue, different online learning techniques have been proposed, for example [1, 8], and many others. [sent-52, score-0.237]

28 Most of current online PCA methods perform the PCs estimation in an incremental manner [8, 18, 25]. [sent-53, score-0.325]

29 Recently, a randomized online PCA algorithm was proposed by [23], whose objective is to minimize the total expected quadratic error minus the total error of the batch algorithm (i. [sent-56, score-0.384]

30 However, none of these online PCA algorithms is robust to the outliers. [sent-59, score-0.348]

31 However, none of them can be directly applied in online learning setting. [sent-65, score-0.237]

32 In [15], an incremental and robust subspace learning method is proposed. [sent-70, score-0.217]

33 In this work, the authors focused on the case where the outliers can be modeled as sparse vectors. [sent-77, score-0.275]

34 However, they also focus on a different case from ours where the outliers are sparse vectors. [sent-82, score-0.275]

35 1 The Algorithm Problem Setup Given a set of observations {y1 , · · · , yT } (here T can be finite or infinite) which are revealed sequentially, the goal of online PCA is to estimate and update the principal components (PCs) based on the newly revealed sample yt at time instance t. [sent-84, score-0.637]

36 Here, the observations are the mixture of authentic samples (non-corrupted samples) and outliers (corrupted samples). [sent-85, score-0.582]

37 The authentic samples zi ∈ Rp are generated through a linear mapping: zi = Axi + ni . [sent-86, score-0.43]

38 The outliers are denoted as oi ∈ Rp and in particular they are defined as follows. [sent-100, score-0.513]

39 j In the above definition, we assume that the basis wj and outliers o are both normalized (see Algorithm 1 step 1)-a) where all the samples are 2 -normalized). [sent-108, score-0.618]

40 In this work, we are interested in the case where the outliers are mixed with authentic samples uniformly in the data stream, i. [sent-112, score-0.55]

41 The input to the proposed online RPCA algorithm is the sequence of observations Y = {y1 , y2 , · · · , yT }, which is the union of authentic samples Z = {zi } generated by the aforementioned linear model and outliers O = {oi }. [sent-115, score-0.836]

42 At the time instance t, online RPCA chooses a set of principal components {wj }d . [sent-118, score-0.36]

43 d T T j=1 wj AA wj Here, wj denotes the true principal components of matrix A. [sent-124, score-0.981]

44 2 Online Robust PCA Algorithm The details of the proposed online RPCA algorithm are shown in Algorithm 1. [sent-136, score-0.254]

45 Since the authentic samples and outliers are mixed uniformly, the outlier fraction in each batch is also λ. [sent-142, score-0.829]

46 Namely, in each batch Bi , there are (1 − λ)b authentic samples and λb outliers. [sent-143, score-0.386]

47 Since the algorithm only involves standard PCA computation, we can employ any incremental or online PCA method [8, 15] to update the PCs estimation upon accepting a new sample. [sent-145, score-0.365]

48 In the algorithm, the initial PC estimation can be obtained through standard PCA or robust PCA [24] on a mini batch of the samples. [sent-148, score-0.307]

49 Perform PCA on the first batch B0 and obtain the initial principal compo(0) nent {wj }d . [sent-158, score-0.255]

50 (t) d j=1 b) Calculate the variance of yi along the direction w(t−1) : δi = (t−1) T wj 2 (t) yi . [sent-166, score-0.368]

51 j=1 (t) ∗ 3) Update the PC as wj = wj , ∀j = 1, . [sent-171, score-0.572]

52 We now explain the intuition of the proposed online RPCA algorithm. [sent-177, score-0.254]

53 Given an initial solution w(0) which is “closer” to the true PC directions than to the outlier direction 1 , the authentic samples will have larger variance along the current PC direction than outliers. [sent-178, score-0.472]

54 Thus in the probabilistic data selection process (as shown in Algorithm 1 step b) to step d)), authentic samples are more likely to be accepted than outliers. [sent-179, score-0.333]

55 Therefore, in the following PC updating based on standard PCA on the accepted data, authentic samples will contribute more than the outliers. [sent-181, score-0.357]

56 4 Main Results In this section we present the theoretical performance guarantee of the proposed online RPCA al(t) gorithm (Algorithm 1). [sent-184, score-0.254]

57 In the sequel, wj is the solution at the t-th time instance. [sent-185, score-0.304]

58 of the true princid pal component wj is j=1 wT AT Awj = 1. [sent-188, score-0.312]

59 2 1) 2 1) (c1 (1 − 2 ) − 4 + −4 2 − λb i=1 c2 (0) T d oi )2 (1 j=1 (wj (1 − λ)b(1 − 2 ) − Γo ) . [sent-195, score-0.238]

60 When the outliers vanish, the second term in the square root of performance H(w(t) ) is zero. [sent-198, score-0.275]

61 To further investigate the behavior of the proposed online RPCA in presence of outliers, we consider the following noiseless case. [sent-208, score-0.287]

62 When the outliers are distributed on the groundtruth subspace, i. [sent-216, score-0.335]

63 , λb d d j=1 |wT oi |2 = 1, the j (0) T conditions become i=1 j=1 (w oi )2 < ∞ and H(w(0) ) ≥ 0. [sent-218, score-0.476]

64 When the outliers are orthogonal to the groundtruth subspace, i. [sent-221, score-0.335]

65 , the conditions for the initial solution becomes H0 ≥ 1/2 − 1/4 − λb i=1 λb i=1 (0) T d oi )2 /(1 j=1 (wj d j=1 |wT oi |2 = 0, j T |w(0) j oi |2 ≤ b(1 − λ)/4, and − λ)b. [sent-223, score-0.781]

66 Hence, when the outlier fraction λ increases, the initial solution should be further away from outliers. [sent-224, score-0.235]

67 When 0 < least 2 d j=1 1/4 − |wT oi |2 < 1, the performance of online RPCA is improved by at j λb i=1 (0) T d oi )2 (1 j=1 (wj − Γo )/(1 − λ)b from its initial solution. [sent-226, score-0.762]

68 Hence, when the initial solution is further away from the outliers, the outlier fraction is smaller, or the outliers are closer to groundtruth subspace, the improvement is more significant. [sent-227, score-0.57]

69 5, the performance of online RPCA still has a positive lower bound. [sent-229, score-0.237]

70 Therefore, the breakdown point of online RPCA is 50%, the highest that any algorithm can achieve. [sent-230, score-0.297]

71 These batch methods are able to provide initial estimate with performance guarantee, which may satisfy the initial condition. [sent-233, score-0.209]

72 5 Proof of The Results We briefly explain the proof of Theorem 1: we first show that when the PCs estimation is being improved, the variance of outliers along the PCs will keep decreasing. [sent-234, score-0.311]

73 Given the result in Lemma 1 , we obtain that the authentic samples concentration properties as stated in the following lemma [24]. [sent-251, score-0.342]

74 We denote the set of accepted authentic samples as Zt and the set of accepted outliers as Ot from the t-th small batch. [sent-260, score-0.666]

75 In the following lemma, we provide the estimation of number of accepted authentic samples |Zt | and outliers |Ot |. [sent-261, score-0.644]

76 For the current obtained principal components {wj authentic samples |Zt | and outliers |Ot | satisfy |Zt | 1 − b b (1−λ)b d (t−1) T (wj i=1 zi )2 ≤ δ and j=1 |Ot | 1 − b b λb d the number of the accepted (t−1) T (wj oi )2 ≤ δ i=1 j=1 2 with probability at least 1 − e−2δ b . [sent-263, score-1.039]

77 In the following lemma, we show that when the algorithm improves the PCs estimation, the impact of outliers will be decreased accordingly. [sent-266, score-0.292]

78 For an outlier oi , an arbitrary orthogonal basis {wj }d and the groundtruth basis j=1 d d d d T T {wj }d which satisfy that j=1 wj oi ≥ wT oi and j=1 wT wj ≥ j j j=1 j=1 j=1 wj oi , the value of d j=1 T wj oi is a monotonically decreasing function of d j=1 w T wj . [sent-268, score-2.81]

79 6 Simulations The numerical study is aimed to illustrate the performance of online robust PCA algorithm. [sent-271, score-0.348]

80 Since it is hard to determine the most adversarial outlier distribution, in simulations, we generate the outliers concentrate on several directions deviating from the groundtruth subspace. [sent-274, score-0.481]

81 This makes a rather adversarial case and is suitable for investigating the robustness of the proposed online RPCA algorithm. [sent-275, score-0.295]

82 Simulation results for p = 100, d = 1, s = 2 and outliers distributed on one direction are shown in Figure 1. [sent-279, score-0.275]

83 5 seconds for the proposed online RPCA to process 10, 000 samples of 100 dimensional, on a PC with Quad CPU with 2. [sent-281, score-0.311]

84 Firstly, online RPCA can improve the PC estimation steadily. [sent-289, score-0.273]

85 Secondly, the performance of online RPCA is rather robust to outliers. [sent-293, score-0.348]

86 To more clearly demonstrate the robustness of online RPCA to outliers, we implement the online PCA proposed in [23] as baseline for the σ = 2 case. [sent-300, score-0.516]

87 The results are presented in Figure 1, from which we can observe that the performance of online PCA drops due to the sensitiveness to newly coming outliers. [sent-301, score-0.341]

88 1, the online PCA cannot recover the true PC directions and the performance is as low as 0. [sent-303, score-0.237]

89 2 50 0 0 10 30 20 # batches 40 50 Figure 1: Performance comparison of online RPCA (blue line) with online PCA (red line). [sent-360, score-0.567]

90 7 Conclusions In this work, we proposed an online robust PCA (online RPCA) algorithm for samples corrupted by outliers. [sent-363, score-0.45]

91 The online RPCA alternates between standard PCA for updating PCs and probabilistic selection of the new samples which alleviates the impact of outliers. [sent-364, score-0.357]

92 Theoretical analysis showed that the online RPCA could improve the PC estimation steadily and provided results with bounded deviation from the optimum. [sent-365, score-0.306]

93 To the best of our knowledge, this is the first work to investigate such online robust PCA problem with theoretical performance guarantee. [sent-366, score-0.348]

94 The proposed online robust PCA algorithm can be applied to handle challenges imposed by the modern big data analysis. [sent-367, score-0.382]

95 A fast algorithm for robust principal components based on projection pursuit. [sent-394, score-0.234]

96 Robpca: a new approach to robust principal component analysis. [sent-443, score-0.232]

97 A fast method for robust principal components with applications to chemometrics. [sent-450, score-0.234]

98 Projection-pursuit approach to robust dispersion matrices and principal components: primary theory and monte carlo. [sent-455, score-0.206]

99 Recursive robust pca or recursive sparse recovery in large but structured noise. [sent-484, score-0.477]

100 Randomized online pca algorithms with regret bounds that are logarithmic in the dimension. [sent-503, score-0.585]


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