nips nips2009 nips2009-87 knowledge-graph by maker-knowledge-mining

87 nips-2009-Exponential Family Graph Matching and Ranking


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Author: James Petterson, Jin Yu, Julian J. Mcauley, Tibério S. Caetano

Abstract: We present a method for learning max-weight matching predictors in bipartite graphs. The method consists of performing maximum a posteriori estimation in exponential families with sufficient statistics that encode permutations and data features. Although inference is in general hard, we show that for one very relevant application–document ranking–exact inference is efficient. For general model instances, an appropriate sampler is readily available. Contrary to existing max-margin matching models, our approach is statistically consistent and, in addition, experiments with increasing sample sizes indicate superior improvement over such models. We apply the method to graph matching in computer vision as well as to a standard benchmark dataset for learning document ranking, in which we obtain state-of-the-art results, in particular improving on max-margin variants. The drawback of this method with respect to max-margin alternatives is its runtime for large graphs, which is comparatively high. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 McAuley and Jin Yu e NICTA, Australian National University Canberra, Australia Abstract We present a method for learning max-weight matching predictors in bipartite graphs. [sent-3, score-0.402]

2 The method consists of performing maximum a posteriori estimation in exponential families with sufficient statistics that encode permutations and data features. [sent-4, score-0.19]

3 Contrary to existing max-margin matching models, our approach is statistically consistent and, in addition, experiments with increasing sample sizes indicate superior improvement over such models. [sent-7, score-0.252]

4 We apply the method to graph matching in computer vision as well as to a standard benchmark dataset for learning document ranking, in which we obtain state-of-the-art results, in particular improving on max-margin variants. [sent-8, score-0.437]

5 This is the problem of finding the ‘heaviest’ perfect match in a weighted bipartite graph. [sent-11, score-0.264]

6 For example, in computer vision the crucial problem of finding a correspondence between sets of image features is often modeled as a matching problem [2, 4]. [sent-14, score-0.225]

7 Ranking algorithms can be based on a matching framework [13], as can clustering algorithms [8]. [sent-15, score-0.183]

8 The problem is that in real applications we typically observe edge feature vectors, not edge weights. [sent-17, score-0.227]

9 In this setting, it is natural to ask whether we could parameterize the features, and use labeled matches in order to estimate the parameters such that, given graphs with ‘similar’ features, their resulting max-weight matches are also ‘similar’. [sent-21, score-0.286]

10 This idea of ‘parameterizing algorithms’ and then optimizing for agreement with data is called structured estimation [27, 29]. [sent-22, score-0.186]

11 [27] and [4] describe max-margin structured estimation formalisms for this problem. [sent-23, score-0.186]

12 Max-margin structured estimators are appealing in that they try to minimize the loss that one really cares about (‘structured losses’, of which the Hamming loss is an example). [sent-24, score-0.353]

13 However structured losses are typically piecewise constant in the parameters, which eliminates any hope of using smooth optimization directly. [sent-25, score-0.153]

14 Max-margin estimators instead minimize a surrogate loss which is easier to optimize, namely a convex upper bound on the structured loss [29]. [sent-26, score-0.403]

15 In practice the results are often good, but known convex relaxations produce estimators which are statistically inconsistent [18], i. [sent-27, score-0.203]

16 1 Motivated by the inconsistency issues of max-margin structured estimators as well as by the wellknown benefits of having a full probabilistic model, in this paper we present a maximum a posteriori (MAP) estimator for the matching problem. [sent-31, score-0.543]

17 The observed data are the edge feature vectors and the labeled matches provided for training. [sent-32, score-0.203]

18 We build an exponential family model where the sufficient statistics are such that the mode of the distribution (the prediction) is the solution of a max-weight matching problem. [sent-34, score-0.329]

19 We then compare the performance of our model instance against a large number of state-of-the-art ranking methods, including DORM [13], an approach that only differs from our model instance by using max-margin instead of a MAP formulation. [sent-37, score-0.223]

20 We show very competitive results on standard document ranking datasets, and in particular we show that our model performs better than or on par with DORM. [sent-38, score-0.398]

21 However the fastest suitable sampler is still quite slow for large models, in which case max-margin matching estimators like those of [4] and [27] are likely to be preferable even in spite of their potential inferior accuracy. [sent-40, score-0.303]

22 1 Background Structured Prediction In recent years, great attention has been devoted in Machine Learning to so-called structured predictors, which are predictors of the kind g✓ : X 7! [sent-42, score-0.189]

23 This structured nature of Y is what structured prediction refers to. [sent-45, score-0.347]

24 In the setting of this paper, X is the set of vector-weighted bipartite graphs (i. [sent-46, score-0.264]

25 , each edge has a feature vector associated with it), and Y is the set of perfect matches induced by X. [sent-48, score-0.27]

26 If N graphs are available, along with corresponding annotated matches (i. [sent-49, score-0.153]

27 , a set {(xn , y n )}N ), our task will be to estimate ✓ such that when we apply n=1 the predictor g✓ to a new graph it produces a match that is similar to matches of similar graphs from the annotated set. [sent-51, score-0.282]

28 Structured learning or structured estimation refers to the process of estimating a vector ✓ for predictor g✓ when data {(x1 , y 1 ), . [sent-52, score-0.186]

29 Two generic estimation strategies have been popular in producing structured predictors. [sent-57, score-0.186]

30 One is based on max-margin estimators [29, 27], and the other on maximum-likelihood (ML) or MAP estimators in exponential family models [12]. [sent-58, score-0.386]

31 However the resulting estimators are known to be inconsistent in general: in the limit of infinite training data the algorithm fails to recover the best model in the model class [18, 16, 15]. [sent-60, score-0.167]

32 The other approach uses ML or MAP estimation in conditional exponential families with ‘structured’ sufficient statistics, such as in probabilistic graphical models, where they are decomposed over the cliques of the graph (in which case they are called Conditional Random Fields, or CRFs [12]). [sent-62, score-0.272]

33 ML and MAP estimators in exponential families not only amount to solving an unconstrained and convex optimization problem; in addition they are statistically consistent. [sent-64, score-0.36]

34 2 The Matching Problem Consider a weighted bipartite graph with m nodes in each part, G = (V, E, w), where V is the set of vertices, E is the set of edges and w : E 7! [sent-68, score-0.307]

35 G can be simply represented by a matrix (wij ) where the entry wij is the weight of the edge ij. [sent-70, score-0.187]

36 Then the matching problem consists of computing 2 QN to attain the graph G = (V, E, w). [sent-81, score-0.265]

37 Therefore, p(✓|Y, X) / p(✓) G = exp log p(✓) + N Y exp (h (xn , y n ), ✓i n=1 N X (h (xn , y n ), ✓i g(x n=1 j i i xij wij = hxij , ✓i j We impose a Gaussian prior on ✓. [sent-84, score-0.262]

38 x is shown, corresponding to the solid |X; ✓), which becomes our log-posterior `(Y is a vector xe associated with each edge with 3 clarity only There is a vector xe ij tion (we suppress edge). [sent-88, score-0.211]

39 Right: weighted associatedgraph G obtained byclarity only Gx onshown, bipartite to each edge e (for evaluating xij is the learned vector ✓ (again the constant term): only edge ij is shown). [sent-89, score-0.499]

40 Right: weighted biparN tite graph G obtained by evaluating Gx on the learned 1 X 2 m X (g(xn ; ✓) h (x `(Y |X; ✓) = k✓k + vector ✓ (again only edge ij is shown). [sent-91, score-0.178]

41 (n) is typical setting consists in each part ofthe scoreis a convex function of ✓ (Wainwright The the number of nodes of engineering g(✓) matrix wij Here M according to domain knowledge and subsequently solving the combinatorial problem. [sent-106, score-0.222]

42 and the other terms are clearly conve 2003) the vector-weighted bipartite graph xn . [sent-107, score-0.409]

43 We then parameterize xij as wiy(i) = f (xiy(i) ; ✓), and the 3. [sent-108, score-0.185]

44 lution of the matching problem (2) to the In this paper we assume that the weights wij are instead to be estimated from training data. [sent-115, score-0.321]

45 More P of the exponential precisely, the weight wij associated with the edge ij in a graph will be the result of an appropriate family model (5), i. [sent-116, score-0.448]

46 Since our goal is to parame h from training composition of a feature vector xij (observed) and Model tures E, x) (x : data). [sent-121, score-0.206]

47 Therefore, in practice, our input is a vector-weighted bipartite graph Gx = (V, of individual pairs of nodes (so as t We assume an exponential family model, where the the to attain an E 7! [sent-122, score-0.508]

48 More formally, assume that a training set See Figure 1 for is M {X, Y } = {(xn , y n )}N is available, where xn := (xn , xn . [sent-125, score-0.335]

49 We then parameterize xij the vector-weighted y), ✓i graph x i=1 ↵ as wiy(i) = f (xiy(i) ; ✓), and the goal is to find the ✓ which maximizes the posterior probability of = ⌦x wiy(i) ⌦ ↵ iy(i) , ✓ , where the observed data. [sent-130, score-0.267]

50 In light of (10), (2) now clea y We assume an exponential family model, where the probability model is a prediction of the best match for Gx under p(y|x; ✓) = exp function, ✓i g(x; convex and (3) is the log-partition(h (x, y), which is a ✓)), where dif✓. [sent-138, score-0.284]

51 Basics y = argmax p(y|x; ✓) = argmax h (x, y), ✓i (5) is the log-partition function, which is a convex and differentiable function of ✓ [31]. [sent-145, score-0.372]

52 ` a convex and di↵erentiable function of ✓ (W and ⇤ML estimation amounts to maximizing the cony = argmax p(y|x; ✓) = argmax h (x, computing (5) & Jordan, 2003), therefore gradient descen ditional likelihood of a sample {X, Y }, i. [sent-147, score-0.405]

53 In practice we will in general introduce a prior exponential families that the gradient o partition function is the expectation of the form MAP estimation: ✓⇤ = argmax p(Y |X; ✓)p(✓) = argmax p(✓|Y, X). [sent-155, score-0.525]

54 3 Feature Parameterization The critical observation now is that we equate the solution of the matching problem (2) to the preP diction of the exponential family model (5), i. [sent-167, score-0.329]

55 Since our goal is to parameterize features of individual pairs of nodes (so as to produce the weight of an edge), the most natural model is M X (x, y) = xiy(i) , which gives (9) i=1 ⌦ ↵ wiy(i) = xiy(i) , ✓ , (10) i. [sent-170, score-0.158]

56 It is a standard result of exponential families that the gradient of the log-partition function is the expectation of the sufficient statistics: r✓ g(x; ✓) = Ey⇠p(y|x;✓) [ (x, y)]. [sent-179, score-0.157]

57 In our experiments, we successfully apply a tractable instance of our model to benchmark document ranking datasets, obtaining very competitive results. [sent-191, score-0.43]

58 The algorithm works by producing exact samples from the distribution of perfect matches on weighted bipartite graphs. [sent-196, score-0.289]

59 K i=1 (15) In our experiments, we apply this algorithm to an image matching application. [sent-200, score-0.225]

60 1 Experiments Ranking Here we apply the general matching model introduced in previous sections to the task of learning to rank. [sent-202, score-0.183]

61 Ranking is a fundamental problem with applications in diverse areas such as document retrieval, recommender systems, product rating and others. [sent-203, score-0.229]

62 Early learning to rank methods applied a pairwise approach, where pairs of documents were used as instances in learning [7, 6, 3]. [sent-204, score-0.229]

63 Recently there has been interest in listwise approaches, where document lists are used as instances, as in our method. [sent-205, score-0.172]

64 In this paper we focus, without loss of generality, on document ranking. [sent-206, score-0.18]

65 We are given a set of queries {qk } and, for each query qk , a list of D(k) documents {dk , . [sent-207, score-0.441]

66 , rD(k) } (assigned by a human editor), measuring the relevance degree of each document with respect to query qk . [sent-213, score-0.485]

67 A rating or relevance degree is usually a nominal value in the list {1, . [sent-214, score-0.198]

68 We are also given, for every k retrieved document dk , a joint feature vector i for that document and the query qk . [sent-218, score-0.713]

69 The subset itself is chosen randomly, provided at least one exemplar document of every rating is present. [sent-223, score-0.261]

70 , dk }, M documents at a time (conditioned on the fact that at least one 1 D(k) exemplar of every rating is present, but otherwise randomly). [sent-228, score-0.289]

71 This effectively boosts the number of training examples since each query qk ends up being selected many times, each time with a different subset of M documents from the original set of D(k) documents. [sent-229, score-0.434]

72 Here we follow the construction used in [13] to map matching problems to ranking problems (indeed the only difference between our ranking model and that of [13] is that they use a max-margin estimator and we use MAP in an exponential family. [sent-231, score-0.84]

73 ) Our edge feature vector xij will be the product of the feature vector i associated with document i, and a scalar cj (the choice of which will be explained below) associated with ranking position j xij = (16) i cj . [sent-232, score-0.953]

74 From (10) and (16), we have wij = cj h i , ✓i, and training proceeds as explained in Section 4. [sent-234, score-0.193]

75 Testing At test time, we are given a query q and its corresponding list of D associated documents. [sent-235, score-0.223]

76 , y ⇤ = argmax y D X⌦ i=1 D X ↵ xiy(i) , ✓ = argmax cy(i) h i , ✓i . [sent-238, score-0.322]

77 4 SortNet 20 hiddens P@10 IsoRank StructRank SortNet 10 hiddens P@10 0. [sent-249, score-0.188]

78 We now notice that if the scalar cj = c(j), where c is a non-increasing function of rank position j, then (17) can be solved simply by sorting the values of h i , ✓i in decreasing order. [sent-265, score-0.168]

79 1 In other words, the matching problem becomes one of ranking the values h i , ✓i. [sent-266, score-0.406]

80 2 In this setting it makes sense to interpret the quantity h i , ✓i as a score of document di for query q. [sent-268, score-0.276]

81 For each query there are a number of associated documents, with relevance degrees judged by humans on three levels: definitely, possibly or not relevant. [sent-277, score-0.256]

82 Again, for each query there are a number of associated documents, with relevance degrees judged by humans, but in this case only two levels are provided: relevant or not relevant. [sent-281, score-0.256]

83 Runtimes for M = 3, 4, 5 are from the ranking experiments, computed by full enumeration; M = 20 corresponds to the image matching experiments, which use the sampler from [10]. [sent-291, score-0.448]

84 Results For the first experiment training was done on subsets sampled as described above, where for each query qk we sampled 0. [sent-305, score-0.337]

85 Also, benchmarking of ranking algorithms is still in its infancy and we don’t yet have publicly available code for all of the competitive methods. [sent-317, score-0.258]

86 Runtime The runtime of our algorithm is competitive with that of max-margin for small graphs, such as those that arise from the ranking application. [sent-325, score-0.294]

87 This is certainly the benefit of the max-margin matching formulations of [4, 13]: it is much faster for large graphs. [sent-327, score-0.183]

88 In this setup, xij = | and i i 2 j | , where | · | denotes the elementwise difference (19) is the Shape Context feature vector [1] for point i. [sent-337, score-0.159]

89 Given the fact that the MAP estimator is consistent while the max-margin estimator is not, one is tempted to investigate the practical performance of both estimators as the sample size grows. [sent-341, score-0.218]

90 Left: Hamming loss for different numbers of training pairs in the image matching problem (test set size fixed to 500 pairs). [sent-363, score-0.367]

91 Right: results of NDCG@1 on the ranking dataset OHSUMED. [sent-364, score-0.223]

92 Left:method for learning max-weight bipartite matching predictors,training pairs (test tensively to to 500 pairs). [sent-367, score-0.421]

93 Right: anwell-known document ranking datasets, larger problems can also results. [sent-368, score-0.363]

94 Wealbeit red example match from the obtaining state-of-the-art be solved, also illustrated–with an image matching application–that test set (blue are correct and slowly, with a recently developed sampler. [sent-369, score-0.272]

95 it consists of performing maximum-a-posteriori estimation in an exponential family model, which results in a simple unconstrained convex optimization problem solvable by standard algorithms such as BFGS. [sent-372, score-0.229]

96 We are going to focus on web page References em we are given a set of queries {qk } and, for each query qk , a list of D(k) documents [1] Belongie, S. [sent-378, score-0.441]

97 Shape matching and object a human editor), measur[2] Belongie, S. [sent-388, score-0.183]

98 R}, where R is k ry retrieved document dk , a joint feature vector i for that document and the query i 8 raining time, we model each query q as a vector-weighted bipartite graph (Figure [4] Caetano, T. [sent-409, score-0.96]

99 Large margin methods for structured and interdependent output variables. [sent-576, score-0.191]

100 A general boosting method and its application to learning ranking functions for web search. [sent-612, score-0.223]


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