nips nips2005 nips2005-195 knowledge-graph by maker-knowledge-mining

195 nips-2005-Transfer learning for text classification


Source: pdf

Author: Chuong B. Do, Andrew Y. Ng

Abstract: Linear text classification algorithms work by computing an inner product between a test document vector and a parameter vector. In many such algorithms, including naive Bayes and most TFIDF variants, the parameters are determined by some simple, closed-form, function of training set statistics; we call this mapping mapping from statistics to parameters, the parameter function. Much research in text classification over the last few decades has consisted of manual efforts to identify better parameter functions. In this paper, we propose an algorithm for automatically learning this function from related classification problems. The parameter function found by our algorithm then defines a new learning algorithm for text classification, which we can apply to novel classification tasks. We find that our learned classifier outperforms existing methods on a variety of multiclass text classification tasks. 1

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 Transfer learning for text classification Chuong B. [sent-1, score-0.204]

2 Ng Computer Science Department Stanford University Stanford, CA 94305 Abstract Linear text classification algorithms work by computing an inner product between a test document vector and a parameter vector. [sent-3, score-0.431]

3 In many such algorithms, including naive Bayes and most TFIDF variants, the parameters are determined by some simple, closed-form, function of training set statistics; we call this mapping mapping from statistics to parameters, the parameter function. [sent-4, score-0.438]

4 Much research in text classification over the last few decades has consisted of manual efforts to identify better parameter functions. [sent-5, score-0.231]

5 The parameter function found by our algorithm then defines a new learning algorithm for text classification, which we can apply to novel classification tasks. [sent-7, score-0.262]

6 We find that our learned classifier outperforms existing methods on a variety of multiclass text classification tasks. [sent-8, score-0.362]

7 1 Introduction In the multiclass text classification task, we are given a training set of documents, each labeled as belonging to one of K disjoint classes, and a new unlabeled test document. [sent-9, score-0.409]

8 Using the training set as a guide, we must predict the most likely class for the test document. [sent-10, score-0.173]

9 “Bag-of-words” linear text classifiers represent a document as a vector x of word counts, and predict the class whose score (a linear function of x) is highest, i. [sent-11, score-0.424]

10 Choosing parameters {θki } which give high classification accuracy on test data, thus, is the main challenge for linear text classification algorithms. [sent-17, score-0.256]

11 In this paper, we focus on linear text classification algorithms in which the parameters are pre-specified functions of training set statistics; that is, each θki is a function θki := g(uki ) of some fixed statistics uki of the training set. [sent-18, score-1.046]

12 Unlike discriminative learning methods, such as logistic regression [1] or support vector machines (SVMs) [2], which use numerical optimization to pick parameters, the learners we consider perform no optimization. [sent-19, score-0.234]

13 Rather, in our technique, parameter learning involves tabulating statistics vectors {u ki } and applying the closed-form function g to obtain parameters. [sent-20, score-0.396]

14 We refer to g, this mapping from statistics to parameters, as the parameter function. [sent-21, score-0.14]

15 Many common text classification methods—including the multinomial and multivariate Bernoulli event models for naive Bayes [3], the vector space-based TFIDF classifier [4], and its probabilistic variant, PrTFIDF [5]—belong to this class of algorithms. [sent-22, score-0.454]

16 Here, picking a good text classifier from this class is equivalent to finding the right parameter function for the available statistics. [sent-23, score-0.315]

17 In practice, researchers often develop text classification algorithms by trial-and-error, guided by empirical testing on real-world classification tasks (cf. [sent-24, score-0.211]

18 In this paper, we consider the task of automatically learning a parameter function g for text classification. [sent-30, score-0.292]

19 Given a set of example text classification problems, we wish to “metalearn” a new learning algorithm (as specified by the parameter function g), which may then be applied new classification problems. [sent-31, score-0.262]

20 In low training data classification tasks, the learning algorithm given by our automatically learned parameter function consistently outperforms human-designed parameter functions based on naive Bayes and TFIDF, as well as existing discriminative learning approaches. [sent-34, score-0.602]

21 A labeled document is a pair (x, y) ∈ X × Y, where x is an n-dimensional vector with xi indicating the number of occurrences of word wi in the document, and y is the document’s class label. [sent-42, score-0.392]

22 A classification problem is a tuple D, S, (xtest , ytest ) , where D is a distribution over X × Y, S = {(xi , yi )}M is a set of M training examples, (xtest , ytest ) is a single test example, and i=1 all M + 1 examples are drawn iid from D. [sent-43, score-0.319]

23 Given a training set S and a test input vector xtest , we must predict the value of the test class label ytest . [sent-44, score-0.52]

24 In linear classification algorithms, we evaluate the score fk (xtest ) := i θki xtest i for assigning xtest to each class k ∈ {1, . [sent-45, score-0.583]

25 , K} and pick the class y = arg maxk fk (xtest ) with the highest score. [sent-48, score-0.162]

26 In our meta-learning setting, we define each θki as the component-wise evaluation of the parameter function g on some vector of training set statistics u ki :     θk1 g(uk1 )  θk2   g(uk2 )   . [sent-49, score-0.439]

27 , n) is a vector whose components are computed from the training set S (we will provide specific examples later). [sent-62, score-0.149]

28 Furthermore, g : Rq → R is the parameter function mapping from uki to its corresponding parameter θki . [sent-63, score-0.775]

29 To illustrate these definitions, we show that two specific cases of the naive Bayes and TFIDF classification methods belong to the class of algorithms described above. [sent-64, score-0.224]

30 Naive Bayes: In the multinomial variant of the naive Bayes classification algorithm, 1 the score for assigning a document x to class k is n NB fk (x) := log p(y = k) + i=1 xi log p(wi | y = k). [sent-65, score-0.596]

31 This view has proved useful for analysis of naive Bayes even when none of its probabilistic assumptions hold [9]; here, we adopt this view, without attaching any particular probabilistic meaning to the empirical frequencies p(·) that happen to be computed by the algorithm. [sent-67, score-0.212]

32 For balanced training sets, the first term is NB irrelevant. [sent-69, score-0.104]

33 2 As before, we write fk (x) = i θki xi with θki = gTFIDF (uki ). [sent-73, score-0.144]

34 The statistics vector is again defined as in (3), but this time, gTFIDF (uki ) := uki1 uki4 log uki5 uki2 2 . [sent-74, score-0.128]

35 These include most other variants of TFIDF based on different TF and IDF terms [7], PrTFIDF [5], and various heuristically modified versions of naive Bayes [6]. [sent-76, score-0.217]

36 3 Learning the parameter function In the last section, we gave two examples of algorithms that obtain their parameters θ ki by applying a function g to a statistics vector uki . [sent-77, score-1.046]

37 In each case, the parameter function was hand-designed, either from probabilistic (in the case of naive Bayes [3]) or geometric (in the case of TFIDF [4]) considerations. [sent-78, score-0.246]

38 We now consider the problem of automatically learning a parameter function from example classification tasks. [sent-79, score-0.119]

39 In the sequel, we assume fixed statistics vectors {uki } and focus on finding an optimal parameter function g. [sent-80, score-0.153]

40 In the standard supervised learning setting, we are given a training set of examples sampled from some unknown distribution D, and our goal is to use the training set to make a prediction on a new test example also sampled from D. [sent-81, score-0.326]

41 By using the training examples to understand the statistical regularities in D, we hope to predict ytest from xtest with low error. [sent-82, score-0.422]

42 Analogously, the problem of meta-learning g is again a supervised learning task; here, however, the training “examples” are now classification problems sampled from a distribution D over classification problems. [sent-83, score-0.169]

43 3 By seeing many instances of text classification problems 2 TFIDF Note that (5) implicitly defines fk (x) as a dot product of two vectors, each of whose components consist of a product of two terms. [sent-84, score-0.317]

44 In the normalized TFIDF classifier, both vectors are normalized to unit length before computing the dot product, a modification that makes the algorithm more stable for documents of varying length. [sent-85, score-0.158]

45 3 Note that in our meta-learning problem, the output of our algorithm is a parameter function g mapping statistics to parameters. [sent-87, score-0.14]

46 Our training data, however, do not explicitly indicate the best parameter function g ∗ for each example classification problem. [sent-88, score-0.14]

47 Effectively then, in the meta-learning task, the central problem is to fit g to some unseen g∗ , based on test examples in each training classification problem. [sent-89, score-0.147]

48 drawn from D, we hope to learn a parameter function g that exploits the statistical regularities in problems from D. [sent-90, score-0.104]

49 For a new, test classification problem Dtest , Stest , (xtest , ytest ) sampled independently from D, we desire that our learned g correctly classify xtest with high probability. [sent-92, score-0.41]

50 Using the linearity assumption, we pose a convex optimization problem for finding a parameter function g that achieves small loss on test examples in the training collection. [sent-94, score-0.229]

51 1 Softmax learning Recall that in softmax regression, the class probabilities p(y | x) are modeled as exp( i θki xi ) , i θk i xi ) k exp( p(y = k | x; {θki }) := k = 1, . [sent-97, score-0.381]

52 , K, (7) where the parameters {θki } are learned from the training data S by maximizing the conditional log likelihood of the data. [sent-100, score-0.21]

53 To pose our optimization problem, we start by learning the linear form g(uki ) = β T uki . [sent-104, score-0.686]

54 Under this parameterization, the conditional likelihood of an example (x, y) is p(y = k | x; β) = exp( k i exp( β T uki xi ) i β T uk i xi ) , k = 1, . [sent-105, score-0.8]

55 (8) In this setup, one natural approach for learning a linear function g is to maximize the (regularized) conditional log likelihood (β : S ) for the entire collection S : m j=1 log p(y (j) | x(j) ; β) − C||β||2   (j) (j) m exp β T i uy(j) i xi  − C||β||2 . [sent-109, score-0.253]

56 = log  (j) (j) exp β T i uki xi j=1 k (β : S ) := (9) In (9), the latter term corresponds to a Gaussian prior on the parameters β, which provides a means for controlling the complexity of the learned parameter function g. [sent-110, score-0.953]

57 The maximization of (9) is similar to softmax regression training except that here, instead of optimizing over the parameters {θki } directly, we optimize over the choice of β. [sent-111, score-0.288]

58 By the Representer Theorem [10], there exists a maximizing solution to (9) for which the optimal parameter vector β ∗ is a linear combination of training set statistics: β∗ = m j=1 k ∗ αjk (j) (j) i uki xi . [sent-114, score-0.877]

59 (10) From this, we reparameterize the original optimization over β in (9) as an equivalent optimization over training example weights {αjk }. [sent-115, score-0.13]

60 For notational convenience, let K(j, j , k, k ) := (j) (j ) i i xi xi (j) (j ) (uki )T uk i . [sent-116, score-0.169]

61 8 1 −30 −35 (b) −30 −25 −20 −15 uk1 −10 −5 0 Figure 1: Distribution of unnormalized uki vectors in dmoz data (a) with and (b) without applying the log transformation in (15). [sent-125, score-0.88]

62 Substituting (10) and (11) into (9), we obtain  m exp ({αjk } : S ) := log  j =1 k exp m m j=1 k αjk K(j, j , k, y (j ) ) m j=1 k αjk K(j, j , k, k ) m −C αjk αj j=1 j =1 k k   K(j, j , k, k ). [sent-129, score-0.123]

63 The assumption that g is a linear function of uki , however, places a severe restriction on the class of learnable parameter functions. [sent-131, score-0.749]

64 4 In particular, choosing a Gaussian (RBF) kernel, K(u, v) := exp(−γ||u − v||2 ), gives a non-parametric representation for g: g(uki ) = β T Φ(uki ) = m j=1 (j) k i (j) αjk xi exp(−γ||uki − uki ||2 ). [sent-133, score-0.704]

65 (14) (j) {αjk xi }, Thus, g(uki ) is a weighted combination of the values where the weights depend (j) exponentially on the squared 2 -distance of uki to each of the statistics vectors {uki }. [sent-134, score-0.799]

66 4 Experiments To validate our method, we evaluated its ability to learn parameter functions on a variety of email and webpage classification tasks in which the number of classes, K, was large (K = 10), and the number of number of training examples per class, m/K, was small (m/K = 2). [sent-136, score-0.241]

67 We used the dmoz Open Directory Project hierarchy,5 the 20 Newsgroups dataset,6 the Reuters-21578 dataset,7 and the Industry Sector dataset8 . [sent-137, score-0.138]

68 gz Table 1: Test set accuracy on dmoz categories. [sent-155, score-0.166]

69 Columns 2-4 give the proportion of correct classifications using non-discriminative methods: the learned g, Naive Bayes, and TFIDF, respectively. [sent-156, score-0.105]

70 Columns 5-7 give the corresponding values for the discriminative methods: softmax regression, 1-vs-all SVMs, and multiclass SVMs. [sent-157, score-0.303]

71 397 The dmoz project is a hierarchical collection of webpage links organized by subject matter. [sent-261, score-0.193]

72 9 Given a dataset of documents, we sampled 10-class 2-training-examples-per-class classification problems by randomly selecting 10 different classes within the dataset, picking 2 training examples within each class, and choosing one test example from a randomly chosen class. [sent-265, score-0.227]

73 For simplicity, we reduced the feature vector in (3) to the following two-dimensional representation,10 uki = log(proportion of wi among words from documents of class k) . [sent-268, score-0.917]

74 log(proportion of documents containing wi ) (15) Note that up to the log transformation, the components of uki correspond to the relative term frequency and document frequency of a word relative to class k (see Figure 1). [sent-269, score-1.126]

75 2 Generalization performance We tested our meta-learning algorithm on classification problems taken from each of the 16 top-level dmoz categories. [sent-271, score-0.161]

76 Dataset gdmoz gnews greut gindu gNB gTFIDF softmax 1VA-SVM MC-SVM dmoz n/a 0. [sent-277, score-0.438]

77 To assess the accuracy of our meta-learning algorithm for a particular test category, we used the g learned from a set of 450 classification problems drawn from the other 15 top-level categories. [sent-310, score-0.145]

78 In 15 out of 16 categories, the learned parameter function g outperforms naive Bayes and TFIDF in addition to the discriminative methods we tested (softmax regression, 1-vs-all SVMs [12], and multiclass SVMs [13]12 ; see Table 1). [sent-312, score-0.47]

79 Here, for each of the four corpora (dmoz, 20 Newsgroups, Reuters-21578, Industry Sector), we constructed independent training and testing datasets of 480 random classification problems. [sent-314, score-0.151]

80 After training separate classifiers (gdmoz , gnews , greut , and gindu ) using data from each of the four corpora, we tested the performance of each learned classifier on the remaining three corpora (see Table 2). [sent-315, score-0.331]

81 Again, the learned parameter functions compare favorably to the other methods. [sent-316, score-0.121]

82 Moreover, these tests show that a single parameter function may give an accurate classification algorithm for many different corpora, demonstrating the effectiveness of our approach for achieving transfer across related learning tasks. [sent-317, score-0.171]

83 5 Discussion and Related Work In this paper, we presented an algorithm based on softmax regression for learning a parameter function g from example classification problems. [sent-318, score-0.271]

84 Another approach for learning g is to modify the multiclass support vector machine formulation of Crammer and Singer [13] in a manner analagous to the modification of softmax regression in Section 3. [sent-320, score-0.372]

85 1, giving the following quadratic program: minimize 1 ||β||2 + C j ξj 2 n m β∈R ,ξ∈R subject to βT (j) i (j) (j) xi (uy(j) i − uki ) ≥ I{k=y(j) } − ξj , ∀k, ∀j. [sent-321, score-0.704]

86 We implemented this method and found that the learned g, like in the softmax formulation, outperforms naive Bayes, TFIDF, and the other discriminative methods. [sent-323, score-0.456]

87 The techniques described in this paper give one approach for achieving inductive transfer in classifier design—using labeled data from related example classification problems to solve a particular classification problem [16, 17]. [sent-324, score-0.151]

88 [18] also consider the issue of knowledge transfer in text classification in the context of ensemble classifiers, and propose a system for using related classification problems to learn the reliability of individual classifiers within the ensemble. [sent-326, score-0.303]

89 The same holdout set was used to select regularization parameters for the discriminative learning algorithms. [sent-328, score-0.153]

90 m/K = 10), softmax and multiclass SVMs consistently outperform naive Bayes and TFIDF; nevertheless, the learned g achieves a performance on par with discriminative methods, despite being constrained to parameters which are explicit functions of training data statistics. [sent-332, score-0.636]

91 This result is consistent with a previous study in which a heuristically hand-tuned version of Naive Bayes attained near-SVM text classification performance for large datasets [6]. [sent-333, score-0.2]

92 Though not directly applied to text classification, Teevan and Karger [19] consider the problem of automatically learning term distributions for use in information retrieval. [sent-335, score-0.256]

93 A similar concept is implicit in the kernelized parameter function learned by our algorithm, where the Gaussian kernel facilitates transfer between similar statistics vectors. [sent-338, score-0.287]

94 A comparison of event models for Naive Bayes text classification. [sent-355, score-0.173]

95 A probabilistic analysis of the Rocchio algorithm with TFIDF for text categorization. [sent-364, score-0.197]

96 Tackling the poor assumptions of naive Bayes text classifiers. [sent-373, score-0.337]

97 generative classifiers: a comparison of logistic regression and naive Bayes. [sent-388, score-0.202]

98 On the algorithmic implementation of multiclass kernel-based vector machines. [sent-417, score-0.133]

99 Inductive transfer for text classification using generalized reliability indicators. [sent-451, score-0.28]

100 Empirical development of an exponential probabilistic model for text retrieval: Using textual analysis to build a better model. [sent-457, score-0.197]


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