nips nips2005 nips2005-105 knowledge-graph by maker-knowledge-mining

105 nips-2005-Large-Scale Multiclass Transduction


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Author: Thomas Gärtner, Quoc V. Le, Simon Burton, Alex J. Smola, Vishy Vishwanathan

Abstract: We present a method for performing transductive inference on very large datasets. Our algorithm is based on multiclass Gaussian processes and is effective whenever the multiplication of the kernel matrix or its inverse with a vector can be computed sufficiently fast. This holds, for instance, for certain graph and string kernels. Transduction is achieved by variational inference over the unlabeled data subject to a balancing constraint. 1

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 au Abstract We present a method for performing transductive inference on very large datasets. [sent-13, score-0.194]

2 Our algorithm is based on multiclass Gaussian processes and is effective whenever the multiplication of the kernel matrix or its inverse with a vector can be computed sufficiently fast. [sent-14, score-0.409]

3 This holds, for instance, for certain graph and string kernels. [sent-15, score-0.204]

4 Transduction is achieved by variational inference over the unlabeled data subject to a balancing constraint. [sent-16, score-0.245]

5 1 Introduction While obtaining labeled data remains a time and labor consuming task, acquisition and storage of unlabelled data is becoming increasingly cheap and easy. [sent-17, score-0.113]

6 This development has driven machine learning research into exploring algorithms that make extensive use of unlabelled data at training time in order to obtain better generalization performance. [sent-18, score-0.116]

7 A common problem of many transductive approaches is that they scale badly with the amount of unlabeled data, which prohibits the use of massive sets of unlabeled data. [sent-19, score-0.312]

8 We perform classification on a dataset consisting of a digraph with 75, 888 vertices and 508, 960 edges. [sent-21, score-0.177]

9 To the best of our knowledge it has so far not been possible to perform transduction on graphs of this size in reasonable time (with standard hardware). [sent-22, score-0.296]

10 Moreover, on graphs various methods for unsupervised learning have been proposed [12, 11], all of which are mainly concerned with computing the kernel matrix on training and test set jointly. [sent-25, score-0.358]

11 Other formulations impose that the label assignment on the test set be consistent with the assumption of confident classification [8]. [sent-26, score-0.138]

12 Yet others impose that training and test set have similar marginal distributions [4]. [sent-27, score-0.222]

13 It is particularly efficient whenever Kα or K −1 α can be computed in linear time, where K ∈ Rm×m is the kernel matrix and α ∈ Rm . [sent-29, score-0.123]

14 • We require consistency of training and test marginals. [sent-30, score-0.115]

15 • Kernels (or their inverses) are computed on training and test set simultaneously. [sent-32, score-0.115]

16 On graphs this can lead to considerable computational savings. [sent-33, score-0.152]

17 • Self consistency of the estimates is achieved by a variational approach. [sent-34, score-0.073]

18 This allows us to make use of Gaussian Process multiclass formulations. [sent-35, score-0.176]

19 2 Multiclass Classification We begin with a brief overview over Gaussian Process multiclass classification [10] recast in terms of exponential families. [sent-36, score-0.176]

20 It is our goal to estimate y|x via p(y|x, θ) = exp ( φ(x, y), θ − g(θ|x)) where g(θ|x) = log exp ( φ(x, y), θ ) . [sent-47, score-0.141]

21 We impose a normal prior on θ, leading to the following negative joint likelihood in θ and Y : m P := − log p(θ, Y |X) = [g(θ|xi ) − φ(xi , yi ), θ ] + i=1 1 θ 2σ 2 2 + const. [sent-49, score-0.185]

22 (2) For transduction purposes p(θ, Y |X) will prove more useful than p(θ|Y, X). [sent-50, score-0.176]

23 Note that a normal prior on θ with variance σ 2 1 implies a Gaussian process on the random variable t(x, y) := φ(x, y), θ with covariance kernel Cov [t(x, y), t(x′ , y ′ )] = σ 2 φ(x, y), φ(x′ , y ′ ) =: σ 2 k((x, y), (x′ , y ′ )). [sent-51, score-0.104]

24 Here joint log-likelihood (2) in terms of α and K yields m n exp ([Kα]iy ) − tr µ⊤ Kα + log i=1 y=1 1 tr α⊤ Kα + const. [sent-60, score-0.354]

25 This is commonly done in Gaussian process literature and we will use both formulations, depending on the problem we need to solve: if Kα can be computed effectively, as is the case with string kernels [9], we use the α-parameterization. [sent-62, score-0.186]

26 Conversely, if K −1 α is cheap, as for example with graph kernels [7], we use the t-parameterization. [sent-63, score-0.222]

27 Derivatives Second order methods such as Conjugate Gradient require the computation of derivatives of − log p(θ, Y |X) with respect to θ in terms of α or t. [sent-64, score-0.09]

28 ∗ (Kβ)) + σ −2 tr γ ⊤ Kβ (8a) 2 ∂t P[u, v] (8b) ⊤ ⊤ = tr u (π. [sent-71, score-0.256]

29 Combining this with rates of convergence for Newton-type or nonlinear CG solver strategies yields overall time costs in the order of O(m log m) to O(m2 ) worst case, a significant improvement over conventional O(m3 ) methods. [sent-77, score-0.095]

30 3 Transductive Inference by Variational Methods As we are interested in transduction, the labels Y (and analogously the data X) decompose as Y = Ytrain ∪ Ytest . [sent-78, score-0.07]

31 Instead, we now aim at estimating the mode of p(θ|X, Ytrain ) by variational means. [sent-80, score-0.073]

32 [5]) − log p(θ|X, Ytrain ) ≤ − log p(θ|X, Ytrain ) + D(q(Ytest ) p(Ytest |X, Ytrain , θ)) =− (log p(Ytest , θ|X, Ytrain ) − log q(Ytest )) q(Ytest ) (9) (10) Ytest holds. [sent-83, score-0.165]

33 The key trick is that while using a factorizing approximation for q we restrict the latter to distributions which satisfy balancing constraints. [sent-85, score-0.122]

34 That is, we require them to yield marginals on the unlabeled data which are comparable with the labeled observations. [sent-86, score-0.106]

35 With qij := q(yi = j) we define µij (q) = qij for all i > mtrain and µij (q) = 1 if yi = 1 and 0 otherwise for all i ≤ mtrain . [sent-89, score-1.258]

36 In other words, we are taking the expectation in µ over all unobserved labels Ytest with respect to the distribution q(Ytest ). [sent-90, score-0.07]

37 We have q(Ytest ) log p(Ytest , θ|X, Ytrain ) Ytest m = n exp ([Kα]ij ) − tr µ(q)⊤ Kα + log i=1 j=1 1 tr α⊤ Kα + const. [sent-91, score-0.409]

38 Since q facm q(Ytest ) log q(Ytest ) = Ytest qij log qij . [sent-95, score-0.834]

39 (13) i=mtrain +1 It is unreasonable to assume that q may be chosen freely from all factorizing distributions (the latter would lead to a straightforward EM algorithm for transductive inference): if we observe a certain distribution of labels on the training set, e. [sent-96, score-0.373]

40 If m ≫ mtrain , however, a naive application of the variational bound can lead to cases where q is concentrated on one class — the increase in likelihood for a resulting very simple classifier completely outweighs any balancing constraints implicit in the data. [sent-100, score-0.496]

41 It is, incidentally, also the reason why SVM transduction optimization codes [4] impose a balancing constraint on the assignment of test labels. [sent-102, score-0.415]

42 We impose the following conditions: m n − rj ≤ + qij ≤ rj for all j ∈ Y and i=mtrain +1 qij = 1 for all i ∈ {mtrain . [sent-103, score-0.949]

43 j=1 − + Here the constraints rj = pemp (y = j) − ǫ and rj = pemp (y = j) + ǫ are chosen such as to correspond to confidence intervals given by finite sample size tail bounds. [sent-106, score-0.36]

44 In other mtrain words we set pemp (y = j) = m−1 train i=1 {yi = j} and ǫ such as to satisfy mtrain m−1 train Pr mtest ξi − m−1 test i=1 ′ ξi > ǫ ≤δ (14) i=1 ′ for iid {0, 1} random variables ξi and ξi with mean p. [sent-107, score-0.713]

45 7)] after application of a union bound over the class labels that ǫ ≤ log(2n/δ)m/ (2mtrain mtest ). [sent-111, score-0.186]

46 4 Graphs, Strings and Vectors We now discuss the two main applications where computational savings can be achieved: graphs and strings. [sent-112, score-0.12]

47 In the case of graphs, the advantage arises from the fact that K −1 is sparse, whereas for texts we can use fast string kernels [9] to compute Kα in linear time. [sent-113, score-0.186]

48 Graphs Denote by G(V, E) the graph given by vertices V and edges E where each edge is a set of two vertices. [sent-114, score-0.172]

49 Then W ∈ R|V |×|V | denotes the adjacency matrix of the graph, where Wij > 0 only if edge {i, j} ∈ E. [sent-115, score-0.101]

50 We assume that the graph G, and thus also the adjacency matrix W , is sparse. [sent-116, score-0.221]

51 Now denote by 1 the identity matrix and by D the diagonal matrix of vertex degrees, i. [sent-117, score-0.108]

52 Then the graph Laplacian and the normalized graph Laplacian of G are given by L := D − W 1 1 ˜ L := 1 − D− 2 W D− 2 , and (15) respectively. [sent-120, score-0.24]

53 Many kernels K (or their inverse) on G are given by low-degree polynomials of the Laplacian or the adjacency matrix of G, such as the following: l l ci W 2i , K = K= i=1 ˜ ˜ (1 − ci L), or K −1 = L + ǫ1. [sent-121, score-0.279]

54 The first kernel arises from an l-step random walk, the third case is typically referred to as regularized graph Laplacian. [sent-123, score-0.189]

55 This means that if the average degree of the graph does not increase with the number of observations, L = O(m) as m = |V | for inference on graphs. [sent-125, score-0.155]

56 From Graphs to Graphical Models Graphs are one of the examples where transduction actually improves computational cost: Assume that we are given the inverse kernel matrix K −1 on training and test set and we wish to perform induction only. [sent-126, score-0.494]

57 In this case we need to compute the kernel matrix (or its inverse) restricted to the training set. [sent-127, score-0.191]

58 Let K −1 = A B , then the upper left hand corner (representing the training set part only) of B⊤ C −1 K is given by the Schur complement A − B ⊤ C −1 B . [sent-128, score-0.101]

59 Moreover, neither the Schur complement nor its inverse are typically sparse. [sent-130, score-0.113]

60 Here we have a nice connection between graphical models and graph kernels. [sent-131, score-0.165]

61 In this case the inverse covariance matrix has nonzero entries only for variables with a direct dependency structure. [sent-133, score-0.134]

62 In other words, if we are given a graphical model of normal random variables, their conditional independence structure is reflected by K −1 . [sent-135, score-0.08]

63 In the same way as in graphical models marginalization may induce dependencies, computing the kernel matrix on the training set only, may lead to dense matrices, even when the inverse kernel on training and test data combined is sparse. [sent-136, score-0.587]

64 The bottom line is there are cases where it is computationally cheaper to take both training and test set into account and optimize over a larger set of variables rather than dealing with a smaller dense matrix. [sent-137, score-0.17]

65 Strings: Efficient computation of string kernels using suffix trees was described in [9]. [sent-138, score-0.186]

66 The efficient computation scheme covers all kernels of type k(x, x′ ) = ws #s (x)#s (x′ ) (17) s for arbitrary ws ≥ 0. [sent-141, score-0.184]

67 This means that computation time for evaluating Kα is again O( i |xi |) as we need to evaluate the kernel expansion for all x ∈ X. [sent-143, score-0.069]

68 Since the average string length is independent of m this yields an O(m) algorithm for Kα. [sent-144, score-0.084]

69 We have: minimize tr q ⊤ τ + q qij log qij (18) i,j − subject to qj ≤ + qij ≤ qj , qij ≥ 0 and i qli = 1 for all j ∈ Y, l ∈ {1. [sent-158, score-1.695]

70 mtest } i Table 1: Error rates on some benchmark datasets (mostly from UCI). [sent-160, score-0.176]

71 The last column is the error rates reported in [1] DATASET cancer cancer (progn. [sent-161, score-0.15]

72 Using Lagrange multipliers one can show that q n needs to satisfy qij = exp(−τij )bi cj where bi , cj ≥ 0. [sent-219, score-0.424]

73 Solving for j qij = 1 yields exp(−τ )c ij j qij = Pn exp(−τil )cl . [sent-220, score-0.809]

74 This means that instead of an optimization problem in mtest × n l=1 variables we now only need to optimize over n variables subject to 2n constraints. [sent-221, score-0.136]

75 + − Note that the exact matching constraint where qi = qi amounts to a maximum likelihood problem for a shifted exponential family model where qij = exp(τij ) exp(γi − gj (γi )). [sent-222, score-0.426]

76 It can be shown that the approximate matching problem is equivalent to a maximum a posteriori optimization problem using the norm dual to expectation constraints on qij . [sent-223, score-0.445]

77 As initialization we choose γi such that the per class averages match the marginal constraint while ignoring the per sample balance. [sent-226, score-0.079]

78 6 Experiments Unfortunately, we are not aware of other multiclass transductive learning algorithms. [sent-228, score-0.369]

79 To still be able to compare our approach to other transductive learning algorithms we performed experiments on some benchmark datasets. [sent-229, score-0.212]

80 To investigate the performance of our algorithm in classifying vertices of a graph, we choose the WebKB dataset. [sent-230, score-0.084]

81 Benchmark datasets Table 1 reports results on some benchmark datasets. [sent-231, score-0.083]

82 To be able to compare the error rates of the transductive multiclass Gaussian Process classifier proposed in this paper, we also report error rates from [2] and an inductive multiclass Gaussian Process classifier. [sent-232, score-0.591]

83 Parameters were chosen by crossvalidation inside the training folds. [sent-234, score-0.134]

84 Graph Mining To illustrate the effectiveness of our approach on graphs we performed experiments on the well known WebKB dataset. [sent-235, score-0.12]

85 This dataset consists of 8275 webpages classified into 7 classes. [sent-236, score-0.117]

86 As we are using this dataset to evaluate our graph mining algorithm, we ignore the text on each webpage and consider the dataset as a labelled directed graph. [sent-238, score-0.335]

87 We use the co-linkage graph and report results for ‘inverse’ 10fold stratified crossvalidations, i. [sent-242, score-0.12]

88 , we use 1 fold as training data and 9 folds as test data. [sent-244, score-0.115]

89 To overcome this, we predict on the test set as follows: For each class the instances that are most likely to be in this class are picked (if they haven’t been picked for a class with lower index) such that the fraction of instances assigned to this class is the same on the training and test set. [sent-247, score-0.382]

90 Although a directed graph approach outperforms there an undirected approach, we resorted to kernels for undirected graphs, as those are computationally more attractive. [sent-250, score-0.258]

91 We will investigate computationally attractive digraph kernels in future work and expect similar benefits as reported by [11]. [sent-251, score-0.222]

92 To investigate the behaviour of our algorithm with less training data, we performed a 20-fold inverse crossvalidation on the ‘wisconsin’ subset and observed an error rate of 17% there. [sent-253, score-0.246]

93 To further strengthen our results and show that the runtime performance of our algorithm is sufficient for classifying the vertices of massive graphs, we also performed initial experiments on the Epinions dataset collected by Mathew Richardson and Pedro Domingos. [sent-254, score-0.156]

94 However, the experiments show that the algorithm can be run on very large graph datasets. [sent-259, score-0.12]

95 7 Discussion and Extensions We presented an efficient method for performing transduction on multiclass estimation problems with Gaussian Processes. [sent-260, score-0.352]

96 It performs particularly well whenever the kernel matrix has special numerical properties which allow fast matrix vector multiplication. [sent-261, score-0.177]

97 That said, also on standard dense problems we observed very good improvements (typically a 10% reduction of the training error) over standard induction. [sent-262, score-0.123]

98 Structured Labels and Conditional Random Fields are a clear area where to extend the transductive setting. [sent-263, score-0.159]

99 The key obstacle to overcome in this context is to find a suitable marginal distribution: with increasing structure of the labels the confidence bounds per subclass decrease dramatically. [sent-264, score-0.116]

100 Learning from labeled and unlabeled data on a o directed graph. [sent-358, score-0.095]


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