nips nips2007 nips2007-53 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Shuheng Zhou, Larry Wasserman, John D. Lafferty
Abstract: Recent research has studied the role of sparsity in high dimensional regression and signal reconstruction, establishing theoretical limits for recovering sparse models from sparse data. In this paper we study a variant of this problem where the original n input variables are compressed by a random linear transformation to m n examples in p dimensions, and establish conditions under which a sparse linear model can be successfully recovered from the compressed data. A primary motivation for this compression procedure is to anonymize the data and preserve privacy by revealing little information about the original data. We characterize the number of random projections that are required for 1 -regularized compressed regression to identify the nonzero coefficients in the true model with probability approaching one, a property called “sparsistence.” In addition, we show that 1 -regularized compressed regression asymptotically predicts as well as an oracle linear model, a property called “persistence.” Finally, we characterize the privacy properties of the compression procedure in information-theoretic terms, establishing upper bounds on the rate of information communicated between the compressed and uncompressed data that decay to zero. 1
Reference: text
sentIndex sentText sentNum sentScore
1 In this paper we study a variant of this problem where the original n input variables are compressed by a random linear transformation to m n examples in p dimensions, and establish conditions under which a sparse linear model can be successfully recovered from the compressed data. [sent-2, score-1.585]
2 A primary motivation for this compression procedure is to anonymize the data and preserve privacy by revealing little information about the original data. [sent-3, score-0.423]
3 We characterize the number of random projections that are required for 1 -regularized compressed regression to identify the nonzero coefficients in the true model with probability approaching one, a property called “sparsistence. [sent-4, score-0.804]
4 ” In addition, we show that 1 -regularized compressed regression asymptotically predicts as well as an oracle linear model, a property called “persistence. [sent-5, score-0.838]
5 ” Finally, we characterize the privacy properties of the compression procedure in information-theoretic terms, establishing upper bounds on the rate of information communicated between the compressed and uncompressed data that decay to zero. [sent-6, score-1.503]
6 It is often important to allow researchers to analyze data without compromising the privacy of customers or leaking confidential information outside the organization. [sent-10, score-0.264]
7 In this paper we show that sparse regression for high dimensional data can be carried out directly on a compressed form of the data, in a manner that can be shown to guard privacy in an information theoretic sense. [sent-11, score-1.192]
8 These compressed data can then be made available for statistical analyses; we focus on the problem of sparse linear regression for high dimensional data. [sent-13, score-0.912]
9 Informally, our theory ensures that the relevant predictors can be learned from the compressed data as well as they could be from the original uncompressed data. [sent-14, score-1.07]
10 However, the original data are not recoverable from the compressed data, and the compressed data effectively reveal no more information than would be revealed by a completely new sample. [sent-16, score-1.486]
11 At the same time, the inference algorithms run faster and require fewer resources than the much larger uncompressed data would require. [sent-17, score-0.312]
12 The data are compressed by a random linear transformation X → X ≡ X , where is a random m × n matrix with m n. [sent-21, score-0.774]
13 Such transformations have been called “matrix masking” in the privacy literature [6]. [sent-23, score-0.28]
14 However, even with = 0 and known, recovering X from X requires solving a highly under-determined linear system and comes with information theoretic privacy guarantees, as we demonstrate. [sent-26, score-0.37]
15 In compressed regression, we assume that the response is also compressed, resulting in the transformed response Y ∈ Rm given by Y → Y ≡ Y = Xβ + = X β + . [sent-28, score-0.74]
16 In the sparse setting, the parameter β ∈ R p is sparse, with a relatively small number s = β 0 of nonzero coefficients in β. [sent-33, score-0.086]
17 The method we focus on is 1 -regularized least squares, also known as the lasso [17]. [sent-34, score-0.352]
18 We study the ability of the compressed lasso estimator to identify the correct sparse set of relevant variables and to predict well. [sent-35, score-1.201]
19 Our first result shows that the lasso is sparsistent under compression, meaning that the correct sparse set of relevant variables is identified asymptotically. [sent-37, score-0.501]
20 Our second result shows that the lasso is persistent under compression. [sent-40, score-0.432]
21 Roughly speaking, persistence [10] means that the procedure predicts well, as measured by the predictive risk R(β) = 2 E Y − β T X , where X ∈ R p is a new input vector and Y is the associated response. [sent-41, score-0.21]
22 P β 1 ≤L n,m R(β) −→ 0, as Our third result analyzes the privacy properties of compressed regression. [sent-45, score-0.997]
23 We evaluate privacy in information theoretic terms by bounding the average mutual information I ( X ; X )/np per matrix entry in the original data matrix X , which can be viewed as a communication rate. [sent-46, score-0.444]
24 Bounding this mutual information is intimately connected with the problem of computing the channel capacity of certain multiple-antenna wireless communication systems [13]. [sent-47, score-0.186]
25 2): The rate at which information about X is revealed by the compressed data X satisfies rn,m = sup I (X ; X ) = O m → 0, where the np n supremum is over distributions on the original data X . [sent-50, score-0.889]
26 As summarized by these results, compressed regression is a practical procedure for sparse learning in high dimensional data that has provably good properties. [sent-51, score-0.896]
27 Analyses of sparsistence, persistence and privacy properties appear in Section 3–5. [sent-53, score-0.386]
28 Simulations for sparsistence and persistence of the compressed lasso are presented in Section 6. [sent-54, score-1.252]
29 2 2 Background and Related Work In this section we briefly review related work in high dimensional statistical inference, compressed sensing, and privacy, to place our work in context. [sent-58, score-0.773]
30 An estimator that has received much attention in the recent literature is the 1 lasso βn [17], defined as βn = arg min 2n Y − Xβ 2 + λn β 1 , where λn is a regularization param2 eter. [sent-60, score-0.5]
31 In [14] it was shown that the lasso is consistent in the high dimensional setting under certain assumptions. [sent-61, score-0.413]
32 d, one cannot simply apply this result to the compressed case. [sent-70, score-0.712]
33 Persistence for the lasso was first defined and studied by Greenshtein and Ritov in [10]; we review their result in Section 4. [sent-71, score-0.352]
34 Compressed regression has close connections to, and draws motivation from compressed sensing [4, 2]. [sent-73, score-0.834]
35 However, in a sense, our motivation is the opposite of compressed sensing. [sent-74, score-0.73]
36 While compressed sensing of X allows a sparse X to be reconstructed from a small number of random measurements, our goal is to reconstruct a sparse function of X . [sent-75, score-0.9]
37 Indeed, from the point of view of privacy, approximately reconstructing X , which compressed sensing shows is possible if X is sparse, should be viewed as undesirable; we return to this point in Section ? [sent-76, score-0.762]
38 Several authors have considered variations on compressed sensing for statistical signal processing tasks [5, 11]. [sent-79, score-0.79]
39 They focus on certain hypothesis testing problems under sparse random measurements, and a generalization to classification of a signal into two or more classes. [sent-80, score-0.097]
40 Research on privacy in statistical data analysis has a long history, going back at least to [3]. [sent-86, score-0.264]
41 The privacy analysis is restricted to observing that recovering X from X requires solving an under-determined linear system. [sent-90, score-0.338]
42 We are not aware of previous work that analyzes the asymptotic properties of a statistical estimator under random projection in the high dimensional setting, giving information-theoretic guarantees, although an information-theoretic quantification of privacy was proposed in [1]. [sent-91, score-0.41]
43 We cast privacy in terms of the rate of information communicated about X through X , maximizing over all distributions on X , and identify this with the problem of bounding the Shannon capacity of a multi-antenna wireless channel, as modeled in [13]. [sent-92, score-0.402]
44 Finally, it is important to mention the active area of cryptographic approaches to privacy from the theoretical computer science community, for instance [9, 7]; however, this line of work is quite different from our approach. [sent-93, score-0.264]
45 The lasso refers to the following: (P1 ) min Y − Xβ 2 such that β 1 ≤ L. [sent-96, score-0.389]
46 In compressed regression we project each column X j ∈ Rn of X to a subspace of m dimensions, using an m × n random projection matrix . [sent-99, score-0.81]
47 Let X = X be the compressed design matrix, and 3 let Y = Y be the compressed response. [sent-100, score-1.443]
48 The compressed lasso is the following optimization problem, for Y = Xβ + = X + , with m being the set of optimal solutions: (a) ( P2 ) min 1 Y − Xβ 2m 2 2 + λm β 1 , (b) m = arg min β∈R p 1 Y − Xβ 2m 2 2 + λm β 1 . [sent-105, score-1.184]
49 (1) Although sparsistency is the primary goal in selecting the correct variables, our analysis establishes conditions for the stronger property of sign consistency: Definition 3. [sent-106, score-0.104]
50 (Sign Consistency) A set of estimators n is sign consistent with the true β if P ∃βn ∈ n s. [sent-108, score-0.08]
51 Clearly, if a set of estimators is sign consistent then it is sparsistent. [sent-112, score-0.08]
52 All recent work establishing results on sparsity recovery assumes some form of incoherence condition on the data matrix X . [sent-113, score-0.168]
53 Assume that X is S -incoherent, where S = supp (β ∗ ), and define s = |S| and ρm = mini∈S |βi∗ |. [sent-137, score-0.091]
54 (4) Then the compressed lasso is sparsistent: P supp (βm ) = supp (β) → 1 as m → ∞. [sent-143, score-1.246]
55 Roughly speaking, persistence implies that a procedure predicts well. [sent-146, score-0.139]
56 We review the arguments in [10] first; we then adapt it to the compressed case. [sent-147, score-0.712]
57 The predictive risk using predictor β T X is R(β) = E(Y − β T X )2 . [sent-150, score-0.097]
58 n (6) Given Bn = {β : β 1 ≤ L n } for L n = o (n/ log n)1/4 , we define the oracle predictor β∗,n = arg min β 1 ≤L n R(β), and the uncompressed lasso estimator βn = arg min β 1 ≤L n Rn (β). [sent-169, score-0.979]
59 Following arguments in [10], it can be shown that under Assumption 1 and given a sequence of sets of estimators Bn = {β : β 1 ≤ L n } for L n = o (n/ log n)1/4 , the sequence of uncompressed P lasso estimators βn = arg min β∈Bn Rn (β) is persistent, i. [sent-173, score-0.93]
60 For the compressed case, again we want to predict (X, Y ), but now the estimator βn,m is based on the lasso from the compressed data of size m n . [sent-177, score-1.821]
61 (7) 1/4 mn Given compressed sample size m n , let Bn,m = {β : β 1 ≤ L n,m }, where L n,m = o log(npn ) . [sent-182, score-0.731]
62 We define the compressed oracle predictor β∗,n,m = arg min β : β 1 ≤L n,m R(β) and the compressed lasso estimator βn,m = arg min β : β 1 ≤L n,m Rn,m (β). [sent-183, score-2.052]
63 The main difference between the sequence of compressed lasso estimators and the original uncompressed sequence is that n and m n together define the sequence of estimators for the compressed data. [sent-188, score-2.278]
64 In Section 6 we illustrate the compressed lasso persistency via simulations to compare the empirical risks with the oracle risks on such a subsequence for a fixed n. [sent-190, score-1.227]
65 5 Information Theoretic Analysis of Privacy Next we derive bounds on the rate at which the compressed data X reveal information about the uncompressed data X . [sent-191, score-1.086]
66 Our general approach is to consider the mapping X → X + as a noisy communication channel, where the channel is characterized by multiplicative noise and additive noise . [sent-192, score-0.14]
67 Since the number of symbols in X is np we normalize by this effective block length to define the information rate rn,m per symbol as rn,m = sup p(X ) I (X ; X ) . [sent-193, score-0.138]
68 Thus, we seek bounds on np the capacity of this channel. [sent-194, score-0.149]
69 A privacy guarantee is given in terms of bounds on the rate rn,m → 0 decaying to zero. [sent-195, score-0.303]
70 Intuitively, if the mutual information satisfies I (X ; X ) = H (X ) − H (X | X ) ≈ 0, then the compressed data X reveal, on average, no more information about the original data X than could be obtained from an independent sample. [sent-196, score-0.756]
71 The underlying channel is equivalent to the multiple antenna model for wireless communication [13], where there are n transmitter and m receiver antennas in a Raleigh flat-fading environment. [sent-197, score-0.219]
72 The propagation coefficients between pairs of transmitter and receiver antennas are modeled by the matrix entries i j ; they remain constant for a coherence interval of p time periods. [sent-198, score-0.137]
73 Computing the 5 channel capacity over multiple intervals requires optimization of the joint density of pn transmitted signals, the problem studied in [13]. [sent-199, score-0.123]
74 Suppose that E[X 2 ] ≤ P and the compressed data are formed by Z = X + γ , j where is m ×n with independent entries i j ∼ N (0, 1/n) and is m × p with independent entries ij ∼ N (0, 1). [sent-203, score-0.76]
75 Then the information rate rn,m satisfies rn,m = sup p(X ) I (X ; Z ) np ≤ m n log 1 + P γ2 . [sent-204, score-0.177]
76 When = 0, or equivalently γ = 0, which is the case assumed in our sparsistence and persistence results, the above analysis yields the trivial bound rn,m ≤ ∞. [sent-206, score-0.188]
77 Suppose that E[X 2 ] ≤ P and the compressed data are formed by Z = X , where j is m × n with independent entries i j ∼ N (0, 1/n). [sent-210, score-0.736]
78 Then the information rate rn,m satisfies m rn,m = sup p(X ) I (X ; Z ) ≤ 2n log (2πe P) . [sent-211, score-0.083]
79 np Under our sparsistency lower bound on m, the above upper bounds are rn,m = O(log(np)/n). [sent-212, score-0.172]
80 We note that these bounds may not be the best possible since they are obtained assuming knowledge of the compression matrix , when in fact the privacy protocol requires that and are not public. [sent-213, score-0.428]
81 We first present results that show the compressed lasso is comparable to the uncompressed lasso in recovering the sparsity pattern of the true linear model. [sent-215, score-1.835]
82 We then show results on persistence that are in close agreement with the theoretical results of Section 4. [sent-216, score-0.122]
83 Here we run simulations to compare the compressed lasso with the uncompressed lasso in terms of the probability of success in recovering the sparsity pattern of β ∗ . [sent-219, score-1.877]
84 A design parameter n is the compression factor f = m , which indicates how much the original data are compressed. [sent-221, score-0.16]
85 The results show that when the compression factor f is large enough, the thresholding behaviors as specified in (8) and (9) for the uncompressed lasso carry over to the compressed lasso, when X is drawn from a Gaussian ensemble. [sent-222, score-1.494]
86 In general, the compression factor f is well below the requirement that we have in Theorem 3. [sent-223, score-0.118]
87 Conversely, if then the probability of recovering the true variables using the lasso approaches zero. [sent-231, score-0.41]
88 In the following simulations, we carry out the lasso using procedure lars(Y, X ) that implements the LARS algorithm of [8] to calculate the full regularization path. [sent-232, score-0.372]
89 For the uncompressed case, we run lars(Y, X ) such that Y = Xβ ∗ + , and for the compressed case we run lars( Y, X ) such that Y = Xβ ∗ + . [sent-233, score-1.024]
90 The results show that the behavior under compression is close to the uncompressed case. [sent-235, score-0.43]
91 Here we solve the following 1 -constrained optimization problem β = arg min β 1 ≤L Y − Xβ 2 directly, based on algorithms described by [15]. [sent-237, score-0.083]
92 0 Figure 1: Plots of the number of samples versus the probability of success for recovering sgn(β ∗ ). [sent-281, score-0.085]
93 In the plots on the right, each curve has a compression factor f ∈ {5, 10, 20, 40, 80, 120} for the compressed lasso, thus n = f m; dashed lines mark θ = 1. [sent-287, score-0.83]
94 46 [19], for the uncompressed lasso in (8) and in (9). [sent-291, score-0.664]
95 18 n=9000, p=128, s=9 14 8 10 12 Risk 16 Uncompressed predictive Compressed predictive Compressed empirical 0 2000 4000 6000 8000 Compressed dimension m Figure 2: Risk versus compressed dimension. [sent-292, score-0.766]
96 , 0)T so ∗ ∗ that βb 1 > L n and βb ∈ Bn , and the uncompressed oracle predictive risk is R = 9. [sent-306, score-0.422]
97 For the compressed lasso, given n and pn , and a varying compressed sample size m, we take the ball Bn,m = {β : β 1 ≤ L n,m } where L n,m = m 1/4 / log(npn ). [sent-315, score-1.467]
98 The compressed lasso estimator βn,m for log2 (npn ) ≤ m ≤ n, is persistent over Bn,m by Theorem 4. [sent-316, score-1.189]
99 On the design and quantification of privacy preserving data mining algorithms. [sent-324, score-0.319]
100 Random projection-based multiplicative data perturbation for privacy preserving distributed data mining. [sent-398, score-0.3]
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