nips nips2010 nips2010-108 knowledge-graph by maker-knowledge-mining

108 nips-2010-Graph-Valued Regression


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Author: Han Liu, Xi Chen, Larry Wasserman, John D. Lafferty

Abstract: Undirected graphical models encode in a graph G the dependency structure of a random vector Y . In many applications, it is of interest to model Y given another random vector X as input. We refer to the problem of estimating the graph G(x) of Y conditioned on X = x as “graph-valued regression”. In this paper, we propose a semiparametric method for estimating G(x) that builds a tree on the X space just as in CART (classification and regression trees), but at each leaf of the tree estimates a graph. We call the method “Graph-optimized CART”, or GoCART. We study the theoretical properties of Go-CART using dyadic partitioning trees, establishing oracle inequalities on risk minimization and tree partition consistency. We also demonstrate the application of Go-CART to a meteorological dataset, showing how graph-valued regression can provide a useful tool for analyzing complex data. 1

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 Graph-Valued Regression Han Liu Xi Chen John Lafferty Larry Wasserman Carnegie Mellon University Pittsburgh, PA 15213 Abstract Undirected graphical models encode in a graph G the dependency structure of a random vector Y . [sent-1, score-0.149]

2 We refer to the problem of estimating the graph G(x) of Y conditioned on X = x as “graph-valued regression”. [sent-3, score-0.145]

3 In this paper, we propose a semiparametric method for estimating G(x) that builds a tree on the X space just as in CART (classification and regression trees), but at each leaf of the tree estimates a graph. [sent-4, score-0.354]

4 We study the theoretical properties of Go-CART using dyadic partitioning trees, establishing oracle inequalities on risk minimization and tree partition consistency. [sent-6, score-1.036]

5 We also demonstrate the application of Go-CART to a meteorological dataset, showing how graph-valued regression can provide a useful tool for analyzing complex data. [sent-7, score-0.146]

6 A common way to study the structure of P is to construct the undirected graph G = (V, E), where the vertex set V corresponds to the p components of the vector Y . [sent-9, score-0.161]

7 In particular, let G(x) be the undirected graph corresponding to P (· | X = x). [sent-14, score-0.161]

8 Then G induces a partition of X , denoted as X1 , . [sent-17, score-0.328]

9 , Xm , where x1 and x2 lie in the same partition element if and only if G(x1 ) = G(x2 ). [sent-20, score-0.318]

10 Graph-valued regression is thus the problem of estimating the partition and estimating the graph within each partition element. [sent-21, score-0.799]

11 We present three different partition-based graph estimators; two that use global optimization, and one based on a greedy splitting procedure. [sent-22, score-0.238]

12 One of the optimization based schemes uses penalized empirical risk minimization, the other uses held-out risk minimization. [sent-23, score-0.399]

13 As we show, both methods enjoy strong theoretical properties under relatively weak assumptions; in particular, we establish oracle inequalities on the excess risk of the estimators, and tree partition consistency (under stronger assumptions) in Section 4. [sent-24, score-0.681]

14 In Section 5 we present experimental results on both synthetic data and a meteorological dataset, demonstrating how graph-valued regression can be an effective tool for analyzing high dimensional data with covariates. [sent-27, score-0.146]

15 One way to estimate G from the sample is the graphical lasso or glasso [13, 5, 1], where one assumes that P is Gaussian with mean µ and covariance matrix Σ. [sent-33, score-0.293]

16 Missing edges in the graph correspond to zero elements in the precision matrix Ω = Σ−1 [12, 4, 7]. [sent-34, score-0.216]

17 A sparse estimate of Ω is obtained by solving Ω = arg min tr(SΩ) − log |Ω| + λ Ω 1 (1) Ω 0 where Ω is positive definite, S is the sample covariance matrix, and Ω 1 = j,k |Ωjk | is the elementwise 1 -norm of Ω. [sent-35, score-0.164]

18 In practice, it seems that the glasso yields reasonable graph estimators even if Y is not Gaussian; however, proving conditions under which this happens is an open problem. [sent-41, score-0.294]

19 We can estimate ΣX , ΣY , and ΣXY by their corresponding sample ΣY X ΣY quantities ΣX , ΣY , and ΣXY , and the marginal precision matrix of X, denoted as ΩX , can be estimated using the glasso. [sent-46, score-0.182]

20 In particular, the conditional covariance matrix of Y | X is ΣY |X = ΣY − ΣY X ΩX ΣXY and a sparse estimate of ΩY |X can be obtained by directly plugging ΣY |X into glasso. [sent-48, score-0.185]

21 However, the estimated graph does not vary with different values of X. [sent-49, score-0.164]

22 h h i=1 i=1 Now we apply glasso in (1) with S = Σ(x) to obtain an estimate of G(x). [sent-56, score-0.176]

23 In this approach, we partition X into finitely many connected regions X1 , . [sent-60, score-0.288]

24 Within each Xj , we apply the glasso to get an estimated graph Gj . [sent-64, score-0.308]

25 We take the partition elements to be recursively defined hyperrectangles. [sent-67, score-0.288]

26 As is well-known, we can then represent the partition by a tree, where each leaf node corresponds to a single partition element. [sent-68, score-0.691]

27 In CART, the leaves are associated with the means within each partition element; while in our case, there will be an estimated undirected graph for each leaf node. [sent-69, score-0.566]

28 The task of graph-valued regression is to find a sparse inverse covariance Ω(x) to estimate Ω(x) for any x ∈ X ; in some situations the graph of Ω(x) is of greater interest than the entries of Ω(x) themselves. [sent-90, score-0.294]

29 We partition X into finitely many connected regions X1 , . [sent-92, score-0.288]

30 , Xm , and within each Xj we apply the glasso to estimate a graph Gj . [sent-95, score-0.291]

31 To find the partition, we restrict ourselves to dyadic splits, as studied by [11, 2]. [sent-97, score-0.303]

32 The primary reason for such a choice is the computational and theoretical tractability of dyadic partition based estimators. [sent-98, score-0.591]

33 Let T denote the set of dyadic partitioning trees (DPTs) defined over X = [0, 1]d , where each DPT T ∈ T is constructed by recursively dividing X by means of axis-orthogonal dyadic splits. [sent-99, score-0.679]

34 If a node is associated to the hyperd rectangle A = l=1 [al , bl ], then after being dyadically split along dimension k, the two children (k) are associated with the sub-hyperrectangles AL = l k [al , bl ] and 2 (k) (k) AR = A\AL . [sent-101, score-0.239]

35 , XmT } the partition of X induced by the leaf nodes of T . [sent-105, score-0.392]

36 For a dyadic integer N = 2K , we define TN to be the collection of all DPTs such that no partition has a side length smaller than 2−K . [sent-106, score-0.591]

37 We denote µT (x) and ΩT (x) as the piecewise constant mean and precision functions associated with T : mT mT µXj · I (x ∈ Xj ) and ΩT (x) = µT (x) = j=1 ΩXj · I (x ∈ Xj ) , j=1 where µXj ∈ Rp and ΩXj ∈ Rp×p are the mean vector and precision matrix for Xj . [sent-108, score-0.177]

38 The penalized empirical risk minimization Go-CART estimator is defined as T , µXj , ΩXj b b mT b j=1 = argminT ∈TN ,µX j ∈Mj ,ΩXj ∈Λj R(T, µT , ΩT ) + pen(T ) [[T ]] log 2+2 log(np) . [sent-120, score-0.346]

39 n where R is defined in (3) and pen(T ) = γn · mT 3 (5) Empirically, we may always set the dyadic integer N to be a reasonably large value; the regularization parameter γn is responsible for selecting a suitable DPT T ∈ TN . [sent-121, score-0.303]

40 The held-out negative log-likelihood risk is then given by Rout (T, µT , ΩT ) = 1 n2 n2 mT tr ΩXj (yi − µXj )(yi − µXj )T − log |ΩXj | · I (xi ∈ Xj ) . [sent-130, score-0.247]

41 The held-out risk minimization Go-CART estimator is T = argminT ∈TN Rout (T, µT , ΩT ). [sent-136, score-0.26]

42 The above procedures require us to find an optimal dyadic partitioning tree within TN . [sent-138, score-0.454]

43 We focus on the held-out risk minimization form as in Definition 2, due to its superior empirical performance. [sent-141, score-0.213]

44 But note that our greedy approach is generic and can easily be adapted to the penalized empirical risk minimization form. [sent-142, score-0.331]

45 First, consider the simple case that we are given a dyadic tree structure T which induces a partition Π(T )={X1 , . [sent-143, score-0.735]

46 For any partition element Xj , we estimate the sample mean using D1 : µXj = n1 i=1 1 I (xi ∈ Xj ) n1 yi · I (xi ∈ Xj ) . [sent-147, score-0.404]

47 i=1 The glasso is then used to estimate a sparse precision matrix ΩXj . [sent-148, score-0.318]

48 More precisely, let ΣXj be the sample covariance matrix for the partition element Xj , given by ΣXj = 1 n1 I (xi ∈ Xj ) i=1 n1 yi − µXj yi − µXj T · I (xi ∈ Xj ) . [sent-149, score-0.509]

49 In practice, we run the full regularization path of the glasso, from large λj , which yields very sparse graph, to small λj , and select the graph that minimizes the held-out negative log-likelihood risk. [sent-151, score-0.156]

50 To further improve the model selection performance, we refit the parameters of the precision matrix after the graph has been selected. [sent-152, score-0.216]

51 That is, to reduce the bias of the glasso, we first estimate the sparse precision matrix using 1 -regularization, and then we refit the Gaussian model without 1 -regularization, but enforcing the sparsity pattern obtained in the first step. [sent-153, score-0.174]

52 The natural, standard greedy procedure starts from the coarsest partition X = [0, 1]d and then computes the decrease in the held-out risk by dyadically splitting each hyperrectangle A along dimension k ∈ {1, . [sent-154, score-0.703]

53 The dimension k∗ that results in the largest decrease in held-out risk is selected, where the change in risk is given by (k) (k) (k) ∆Rout (A, µA , ΩA ) = Rout (A, µA , ΩA ) − Rout (AL , µA(k) , ΩA(k) ) − Rout (AR , µA(k) , ΩA(k) ). [sent-158, score-0.381]

54 L L R R If splitting any dimension k of A leads to an increase in the held-out risk, the element A should no longer be split and hence becomes a partition element of Π(T ). [sent-159, score-0.475]

55 This greedy partitioning method parallels the classical algorithms for classification and regression that have been used in statistical learning for decades. [sent-161, score-0.16]

56 4 4 Theoretical Properties We define the oracle risk R∗ over TN as R∗ = R(T ∗ , µ∗ , Ω∗ ) = T T inf T ∈TN ,µXj ∈Mj ,ΩXj ∈Λj R(T, µT , ΩT ). [sent-164, score-0.295]

57 Note that T ∗ , µ∗ ∗ , and Ω∗ ∗ might not be unique, since the finest partition always achieves the oracle T T risk. [sent-165, score-0.344]

58 Let T ∈ TN be an arbitrary DPT which induces a partition Π(T ) = {X1 , . [sent-168, score-0.328]

59 Let T ∈ TN be a DPT that induces a partition Π(T ) = {X1 , . [sent-183, score-0.328]

60 For any δ ∈ (0, 1/4), let T , µT , ΩT be the estimator obtained using the penalized empirical risk minimizab b tion Go-CART in Definition 1, with a penalty term pen(T ) of the form [[T ]] log 2 + 2 log p + log(48/δ) pen(T ) = (C1 + 1)Ln mT n √ √ where C1 = 8 v2 + 8B v1 + B 2 . [sent-187, score-0.339]

61 Then for sufficiently large n, the excess risk inequality R(T , µT , ΩT ) − R∗ ≤ inf b b T ∈TN 2pen(T ) + inf µXj ∈Mj ,ΩXj ∈Λj (R(T, µT , ΩT ) − R∗ ) holds with probability at least 1 − δ. [sent-188, score-0.34]

62 A similar oracle inequality holds when using the held-out risk minimization Go-CART. [sent-189, score-0.269]

63 Let T ∈ TN be a DPT which induces a partition Π(T ) = {X1 , . [sent-191, score-0.328]

64 Let T , µT , ΩT be the estimator constructed using the held-out risk minimization criterion of b b Definition 2. [sent-203, score-0.26]

65 Then, for sufficiently large n, the excess risk inequality R(T , µT , ΩT ) − R∗ ≤ inf b b T ∈TN 3φn (T ) + inf µXj ∈Mj ,ΩXj ∈Λj (R(T, µT , ΩT ) − R∗ ) + φn (T ) with probability at least 1 − δ. [sent-204, score-0.34]

66 We now temporarily make the strong assumption that the model is correct, so that Y given X is conditionally Gaussian, with a partition structure that is given by a dyadic tree. [sent-207, score-0.618]

67 We show that with high probability, the true dyadic partition structure can be correctly recovered. [sent-208, score-0.591]

68 The true model is Y | X = x ∼ Np (µ∗ ∗ (x), Ω∗ ∗ (x)) T T ∗ where T ∈ TN is a DPT with induced partition Π(T ∗ ) = {Xj∗ }mT ∗ and j=1 µ∗ ∗ (x) = T mT ∗ µ∗ I(x ∈ Xj∗ ), j Ω∗ ∗ (x) = T j=1 Ω∗ I(x ∈ Xj∗ ). [sent-210, score-0.324]

69 A tree estimation procedure T is tree partition consistent in case P Π(T ∗ ) ⊂ Π(T ) → 1 as n → ∞. [sent-214, score-0.496]

70 Note that the estimated partition may be finer than the true partition. [sent-215, score-0.337]

71 Establishing a tree partition consistency result requires further technical assumptions. [sent-216, score-0.392]

72 The following assumption specifies that for arbitrary adjacent subregions of the true dyadic partition, either the means or the variances should be sufficiently different. [sent-217, score-0.508]

73 Let Xi∗ and Xj∗ be adjacent partition elements of T ∗ , so that they have a common parent node within T ∗ . [sent-220, score-0.335]

74 Moreover, the Go-CART estimator in both the penalized risk minimization and held-out risk minimization form is tree partition consistent. [sent-230, score-0.918]

75 inf Π(T ∗ ) inf Π(T ) µT , ΩT ∈MT R(T, µT , ΩT ) − R(T ∗ , µ∗ ∗ , Ω∗ ∗ ) > min{ T T This result shows that, with high probability, we obtain a finer partition than T ∗ ; the assumptions do not, however, control the size of the resulting partition. [sent-231, score-0.42]

76 5 Experiments We now present the performance of the greedy partitioning algorithm of Section 3 on both synthetic data and a real meteorological dataset. [sent-233, score-0.21]

77 In the experiment, we always set the dyadic integer N = 210 to ensure that we can obtain fine-tuned partitions of the input space X . [sent-234, score-0.342]

78 (a) Estimated dyadic tree structure; (b) Ground true partition. [sent-289, score-0.407]

79 The number labeled on each subregion corresponds to each leaf node ID of the tree in (a); (c) The held-out negative log-likelihood risk for each split. [sent-293, score-0.518]

80 The order of the splits corresponds the ID of the tree node (from small to large). [sent-294, score-0.178]

81 hypercube into 22 subregions as shown in Figure 1 (b). [sent-295, score-0.21]

82 For the t-th subregion where 1 ≤ t ≤ 22, we generate an Erd¨ s-R´ nyi random graph Gt = (V t , E t ) with the number of vertices p = 20, o e the number of edges |E| = 10 and the maximum node degree is four. [sent-296, score-0.288]

83 The learned dyadic tree structure and its induced partition are presented in Figure 1. [sent-302, score-0.731]

84 Even though the graphs within each subregion are sparse, the estimated graph obtained by pooling all the data together is highly dense. [sent-306, score-0.361]

85 As the greedy algorithm proceeds, the estimated graphs become sparser and sparser. [sent-307, score-0.212]

86 Out of the 82 simulations where we correctly identify the tree structure, we list the graph estimation performance for subregions 28, 29, 13, 14, 5, 6 in terms of precision, recall, and F1-score in Table 1. [sent-309, score-0.397]

87 Table 1: The graph estimation performance over different subregions Mean values over 100 runs (Standard deviation) subregion region 28 region 29 region 13 region 14 region 5 region 6 Precision Recall F1 − score 0. [sent-310, score-0.611]

88 We also plot the held-out risk in the subplot (c). [sent-351, score-0.212]

89 The whole risk curve illustrates a diminishing return behavior. [sent-353, score-0.173]

90 Correctly splitting the large rectangle leads to a significant decrease in the risk; in contrast, splitting the middle rectangles does not reduce the risk as much. [sent-354, score-0.315]

91 (a) Learned partitions for the 100 locations and projected to the US map, with the estimated graphs for subregions 3, 10, and 33; (b) Estimated graph with data pooled from all 100 locations; (c) the re-scaled partition pattern induced by the learned dyadic tree structure. [sent-357, score-1.267]

92 As a baseline, we estimate a sparse graph on the data pooled from all 100 locations, using the glasso algorithm; the estimated graph is shown in Figure 2 (b). [sent-368, score-0.542]

93 This apparently contradicts the basic domain knowledge that CO2 should be correlated with the solar radiation factors (including GLO, DIR), according to the IPCC report [6] which is one of the most authoritative reports in the field of meteorology. [sent-370, score-0.156]

94 Treating the longitude and latitude of each site as two-dimensional covariate X, and the meteorology data of the p = 15 factors as the response Y , we estimate a dyadic tree structure using the greedy algorithm. [sent-372, score-0.543]

95 The result is a partition with 66 subregions, shown in Figure 2. [sent-373, score-0.288]

96 The graphs for subregions 3 and 10 (corresponding to the coast of California and Arizona states) are shown in subplot (a) of Figure 2. [sent-374, score-0.288]

97 The graphs for these two adjacent subregions are quite similar, suggesting spatial smoothness of the learned graphs. [sent-375, score-0.249]

98 Moreover, for both graphs, CO2 is connected to the solar radiation factor GLO through CH4 . [sent-376, score-0.156]

99 In contrast, for subregion 33, which corresponds to the north part of Arizona, the estimated graph is quite different. [sent-377, score-0.29]

100 In general, it is found that the graphs corresponding to the locations along the coasts are sparser than those corresponding to the locations in the mainland. [sent-378, score-0.174]


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