nips nips2013 nips2013-99 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Stefan Wager, Sida Wang, Percy Liang
Abstract: Dropout and other feature noising schemes control overfitting by artificially corrupting the training data. For generalized linear models, dropout performs a form of adaptive regularization. Using this viewpoint, we show that the dropout regularizer is first-order equivalent to an L2 regularizer applied after scaling the features by an estimate of the inverse diagonal Fisher information matrix. We also establish a connection to AdaGrad, an online learning algorithm, and find that a close relative of AdaGrad operates by repeatedly solving linear dropout-regularized problems. By casting dropout as regularization, we develop a natural semi-supervised algorithm that uses unlabeled data to create a better adaptive regularizer. We apply this idea to document classification tasks, and show that it consistently boosts the performance of dropout training, improving on state-of-the-art results on the IMDB reviews dataset. 1
Reference: text
sentIndex sentText sentNum sentScore
1 edu Abstract Dropout and other feature noising schemes control overfitting by artificially corrupting the training data. [sent-4, score-0.575]
2 For generalized linear models, dropout performs a form of adaptive regularization. [sent-5, score-0.76]
3 Using this viewpoint, we show that the dropout regularizer is first-order equivalent to an L2 regularizer applied after scaling the features by an estimate of the inverse diagonal Fisher information matrix. [sent-6, score-0.931]
4 By casting dropout as regularization, we develop a natural semi-supervised algorithm that uses unlabeled data to create a better adaptive regularizer. [sent-8, score-0.938]
5 We apply this idea to document classification tasks, and show that it consistently boosts the performance of dropout training, improving on state-of-the-art results on the IMDB reviews dataset. [sent-9, score-0.756]
6 1 Although dropout has proved to be a very successful technique, the reasons for its success are not yet well understood at a theoretical level. [sent-12, score-0.718]
7 For example, Bishop [9] showed that the effect of training with features that have been corrupted with additive Gaussian noise is equivalent to a form of L2 -type regularization in the low noise limit. [sent-15, score-0.314]
8 In this paper, we take a step towards understanding how dropout training works by analyzing it as a regularizer. [sent-16, score-0.782]
9 We focus on generalized linear models (GLMs), a class of models for which feature dropout reduces to a form of adaptive model regularization. [sent-17, score-0.818]
10 Using this framework, we show that dropout training is first-order equivalent to L2 -regularization afˆ ˆ ter transforming the input by diag(I) 1/2 , where I is an estimate of the Fisher information matrix. [sent-18, score-0.801]
11 In the case of logistic regression, dropout can be interpreted as a form of adaptive L2 -regularization that favors rare but useful features. [sent-20, score-0.98]
12 introduced dropout training in the context of neural networks specifically, and also advocated omitting random hidden layers during training. [sent-31, score-0.804]
13 In this paper, we follow [2, 3] and study feature dropout as a generic training method that can be applied to any learning algorithm. [sent-32, score-0.84]
14 1 1 and dropout training have an intimate connection: Just as SGD progresses by repeatedly solving linearized L2 -regularized problems, a close relative of AdaGrad advances by solving linearized dropout-regularized problems. [sent-33, score-0.888]
15 Our formulation of dropout training as adaptive regularization also leads to a simple semi-supervised learning scheme, where we use unlabeled data to learn a better dropout regularizer. [sent-34, score-1.772]
16 We apply this idea to several document classification problems, and find that it consistently improves the performance of dropout training. [sent-36, score-0.736]
17 On the benchmark IMDB reviews dataset introduced by [12], dropout logistic regression with a regularizer tuned on unlabeled data outperforms previous state-of-the-art. [sent-37, score-1.165]
18 2 Artificial Feature Noising as Regularization We begin by discussing the general connections between feature noising and regularization in generalized linear models (GLMs). [sent-39, score-0.563]
19 We will apply the machinery developed here to dropout training in Section 4. [sent-40, score-0.782]
20 A GLM defines a conditional distribution over a response y 2 Y given an input feature vector x 2 Rd : def p (y | x) = h(y) exp{y x · A(x · )}, def `x,y ( ) = log p (y | x). [sent-41, score-0.186]
21 Given n training examples (xi , yi ), the standard maximum likelihood estimate ˆ 2 Rd minimizes the empirical loss over the training examples: ˆ def arg min = 2Rd n X (2) `xi , yi ( ). [sent-49, score-0.312]
22 i=1 With artificial feature noising, we replace the observed feature vectors xi with noisy versions xi = ˜ ⌫(xi , ⇠i ), where ⌫ is our noising function and ⇠i is an independent random variable. [sent-50, score-0.675]
23 In other words, dropout noise corresponds to setting xij ˜ to 0 with probability and to xij /(1 ) else. [sent-55, score-0.849]
24 2 Integrating over the feature noise gives us a noised maximum likelihood parameter estimate: ˆ = arg min 2Rd n X i=1 def E⇠ [`xi , yi ( )] , where E⇠ [Z] = E [Z | {xi , yi }] ˜ (3) is the expectation taken with respect to the artificial feature noise ⇠ = (⇠1 , . [sent-56, score-0.417]
25 For GLMs, the noised empirical loss takes on a simpler form: n X E⇠ [`xi , yi ( )] = ˜ i=1 2 Artificial noise of the form xi to dropout noise as defined by [1]. [sent-61, score-0.986]
26 The key observation here is that the effect of artificial feature noising reduces to a penalty R( ) that does not depend on the labels {yi }. [sent-68, score-0.619]
27 Because of this, artificial feature noising penalizes the complexity of a classifier in a way that does not depend on the accuracy of a classifier. [sent-69, score-0.55]
28 Thus, for GLMs, artificial feature noising is a regularization scheme on the model itself that can be compared with other forms of regularization such as ridge (L2 ) or lasso (L1 ) penalization. [sent-70, score-0.647]
29 In Section 6, we exploit the label-independence of the noising penalty and use unlabeled data to tune our estimate of R( ). [sent-71, score-0.739]
30 It is easy to verify that these expressions are in general not equivalent, but ˆ they are equivalent when the effect of feature noising reduces to a label-independent penalty on the likelihood. [sent-75, score-0.663]
31 1 A Quadratic Approximation to the Noising Penalty Although the noising penalty R yields an explicit regularizer that does not depend on the labels {yi }, the form of R can be difficult to interpret. [sent-78, score-0.635]
32 Applying this quadratic approximation to (5) x x yields the following quadratic noising regularizer, which will play a pivotal role in the rest of the paper: n X def 1 Rq ( ) = A00 (xi · ) Var⇠ [˜i · ] . [sent-82, score-0.665]
33 x (6) 2 i=1 This regularizer penalizes two types of variance over the training examples: (i) A00 (xi · ), which corresponds to the variance of the response yi in the GLM, and (ii) Var⇠ [˜i · ], the variance of the x estimated GLM parameter due to noising. [sent-83, score-0.193]
34 3 Accuracy of approximation Figure 1a compares the noising penalties R and Rq for logistic redef gression in the case that x · is Gaussian;4 we vary the mean parameter p = (1 + e x· ) 1 and the ˜ q noise level . [sent-84, score-0.637]
35 atheism vs 3 Although Rq is not convex, we were still able (using an L-BFGS algorithm) to train logistic regression with Rq as a surrogate for the dropout regularizer without running into any major issues with local optima. [sent-91, score-1.011]
36 4 This assumption holds a priori for additive Gaussian noise, and can be reasonable for dropout by the central limit theorem. [sent-92, score-0.765]
37 5 0 50 Sigma 100 150 Training Iteration (a) Comparison of noising penalties R and Rq for logistic regression with Gaussian perturbations, i. [sent-109, score-0.649]
38 The solid line indicates the true penalty and the dashed one is our quadratic approximation thereof; p = (1 + e x· ) 1 is the mean parameter for the logistic model. [sent-112, score-0.293]
39 (b) Comparing the evolution of the exact dropout penalty R and our quadratic approximation Rq for logistic regression on the AthR classification task in [15] with 22K features and n = 1000 examples. [sent-113, score-1.13]
40 In practice, we have found that fitting logistic regression with the quadratic surrogate Rq gives similar results to actual dropout-regularized logistic regression. [sent-120, score-0.368]
41 3 Regularization based on Additive Noise Having established the general quadratic noising regularizer Rq , we now turn to studying the effects of Rq for various likelihoods (linear and logistic regression) and noising models (additive and dropout). [sent-122, score-1.147]
42 In this section, we warm up with additive noise; in Section 4 we turn to our main target of interest, namely dropout noise. [sent-123, score-0.765]
43 Applying these facts to (6) yields a simplified form for the quadratic noising penalty: 1 2 Rq ( ) = nk k2 . [sent-126, score-0.518]
44 (7) 2 2 Thus, we recover the well-known result that linear regression with additive feature noising is equivalent to ridge regression [2, 9]. [sent-127, score-0.755]
45 For logistic regression, A00 (xi · ) = pi (1 pi ) where pi = (1 + exp( xi · )) 1 is the predicted probability of yi = 1. [sent-130, score-0.518]
46 The quadratic noising penalty is then 1 R ( )= 2 q 2 k k2 2 n X pi (1 pi ). [sent-131, score-0.842]
47 (8) i=1 In other words, the noising penalty now simultaneously encourages parsimonious modeling as before (by encouraging k k2 to be small) as well as confident predictions (by encouraging the pi ’s to 2 move away from 1 ). [sent-132, score-0.691]
48 The pi = (1 + e xi · ) 1 are mean ij parameters for the logistic model. [sent-137, score-0.29]
49 We can check that: d X 1 2 Var⇠ [˜i · ] = x x2 j , (9) ij 21 j=1 and so the quadratic dropout penalty is Rq ( ) = 1 21 n X i=1 A00 (xi · ) d X x2 ij 2 j. [sent-139, score-0.945]
50 Thus, nX V ( dropout can be seen as an attempt to apply an L2 penalty after normalizing the feature vector by diag(I) 1/2 . [sent-143, score-0.905]
51 Linear Regression For linear regression, V is the identity matrix, so the dropout objective is equivalent to a form of ridge regression where each column of the design matrix is normalized before applying the L2 penalty. [sent-149, score-0.859]
52 Logistic Regression The form of dropout penalties becomes much more intriguing once we move beyond the realm of linear regression. [sent-151, score-0.739]
53 Here, we can write the quadratic dropout penalty from (10) as n d XX 1 2 Rq ( ) = pi (1 pi ) x2 j . [sent-153, score-1.107]
54 (12) ij 21 i=1 j=1 Thus, just like additive noising, dropout generally gives an advantage to confident predictions and small . [sent-154, score-0.814]
55 However, unlike all the other methods considered so far, dropout may allow for some large 2 pi (1 pi ) and some large j , provided that the corresponding cross-term x2 is small. [sent-155, score-0.934]
56 ij Our analysis shows that dropout regularization should be better than L2 -regularization for learning weights for features that are rare (i. [sent-156, score-0.943]
57 , often 0) but highly discriminative, because dropout effectively does not penalize j over observations for which xij = 0. [sent-158, score-0.762]
58 Thus, in order for a feature to earn a large 2 pi ) each time that it j , it suffices for it to contribute to a confident prediction with small pi (1 is active. [sent-159, score-0.274]
59 6 To be precise, dropout does not reward all rare but discriminative features. [sent-161, score-0.862]
60 Rather, dropout rewards those features that are rare and positively co-adapted with other features in a way that enables the model to make confident predictions whenever the feature of interest is active. [sent-162, score-0.99]
61 5 Table 3: Accuracy of L2 and dropout regularized logistic regression on a simulated example. [sent-163, score-0.893]
62 55 classification where rare but discriminative features are prevalent [3]. [sent-175, score-0.19]
63 We summarize the relationship between L2 -penalization, additive noising and dropout in Table 2. [sent-177, score-1.218]
64 Additive noising introduces a product-form penalty depending on both and A00 . [sent-178, score-0.561]
65 However, the full potential of artificial feature noising only emerges with dropout, which allows the penalty terms due to and A00 to interact in a non-trivial way through the design matrix X (except for linear regression, in which all the noising schemes we consider collapse to ridge regression). [sent-179, score-1.121]
66 1 A Simulation Example The above discussion suggests that dropout logistic regression should perform well with rare but useful features. [sent-181, score-0.993]
67 To make sure that our experiment was picking up the effect of dropout training specifically and not just normalization of X, we ensured that the columns of X were normalized in expectation. [sent-184, score-0.782]
68 The dropout penalty for logistic regression can be written as a matrix product 0 10 1 ··· ··· 1 2 Rq ( ) = (· · · pi (1 pi ) · · ·) @· · · x2 · · ·A @ j A . [sent-185, score-1.217]
69 ij 21 ··· ··· (13) We designed the simulation study in such a way that, at the optimal , the dropout penalty should have structure Small (confident prediction) Big (weak prediction) ! [sent-186, score-0.873]
70 C A (14) A dropout penalty with such a structure should be small. [sent-188, score-0.826]
71 Although there are some uncertain pre2 dictions with large pi (1 pi ) and some big weights j , these terms cannot interact because the 2 corresponding terms xij are all 0 (these are examples without any of the rare discriminative features and thus have no signal). [sent-189, score-0.475]
72 Our simulation results, given in Table 3, confirm that dropout training outperforms L2 -regularization here as expected. [sent-191, score-0.802]
73 In SGD, the weight vector ˆ is updated with ˆt+1 = ˆt ⌘t gt , where gt = r`xt , yt ( ˆt ) is the gradient of the loss due to the t-th training example. [sent-195, score-0.207]
74 Left: 10000 labeled training examples, and up to 40000 unlabeled examples. [sent-211, score-0.278]
75 Right: 3000-15000 labeled training examples, and 25000 unlabeled examples. [sent-212, score-0.278]
76 At least superficially, AdaGrad and dropout seem to have similar goals: For logistic regression, they can both be understood as adaptive alternatives to methods based on L2 -regularization that favor learning rare, useful features. [sent-220, score-0.862]
77 (17) This implies that dropout descent is first-order equivalent to an adaptive SGD procedure with At = diag(Ht ). [sent-223, score-0.796]
78 Thus, by using dropout instead of L2 -regularization to solve linearized problems in online learning, we end up with an AdaGrad-like algorithm. [sent-226, score-0.774]
79 Of course, the connection between AdaGrad and dropout is not perfect. [sent-227, score-0.751]
80 But, at a high level, AdaGrad and dropout appear to both be aiming for the same goal: scaling the features by the Fisher information to make the level-curves of the objective more circular. [sent-229, score-0.764]
81 In the case of logistic regression, AROW also favors learning rare features, but unlike dropout and AdaGrad does not privilege confident predictions. [sent-231, score-0.938]
82 Table 4: Performance of semi-supervised dropout training for document classification. [sent-233, score-0.8]
83 (a) Test accuracy with and without unlabeled data on different datasets. [sent-234, score-0.201]
84 : logistic regression with L2 regularization; Dropout: dropout trained with quadratic surrogate; +Unlabeled: using unlabeled data. [sent-238, score-1.136]
85 Labeled: using just labeled data from each paper/method, +Unlabeled: use additional unlabeled data. [sent-240, score-0.214]
86 Drop: dropout with Rq , MNB: multionomial naive Bayes with semisupervised frequency estimate from [19],8 -Uni: unigram features, -Bi: bigram features. [sent-241, score-0.739]
87 As a result, we can use additional unlabeled training examples to estimate it more accurately. [sent-275, score-0.267]
88 Suppose we have an unlabeled dataset {zi } of size m, and let ↵ 2 (0, 1] be a discount factor for the unlabeled data. [sent-276, score-0.356]
89 Then we can define a semi-supervised penalty estimate ⌘ n ⇣ def R⇤ ( ) = R( ) + ↵ RUnlabeled ( ) , (19) n + ↵m P where R( ) is the original penalty estimate and RUnlabeled ( ) = i E⇠ [A(zi · )] A(zi · ) is computed using (5) over the unlabeled examples zi . [sent-277, score-0.502]
90 Our semi-supervised approach is based on a different intuition: we’d like to set weights to make confident predictions on unlabeled data as well as the labeled data, an intuition shared by entropy regularization [24] and transductive SVMs [25]. [sent-284, score-0.306]
91 Overall, we see that using unlabeled data to learn a better regularizer R⇤ ( ) consistently improves the performance of dropout training. [sent-287, score-0.97]
92 The dataset contains 50,000 unlabeled examples in addition to the labeled train and test sets of size 25,000 each. [sent-289, score-0.257]
93 Whereas the train and test examples are either positive or negative, the unlabeled examples contain neutral reviews as well. [sent-290, score-0.266]
94 We train a dropout-regularized logistic regression classifier on unigram/bigram features, and use the unlabeled data to tune our regularizer. [sent-291, score-0.371]
95 Our method benefits from unlabeled data even in the presence of a large amount of labeled data, and achieves state-of-the-art accuracy on this dataset. [sent-292, score-0.237]
96 7 Conclusion We analyzed dropout training as a form of adaptive regularization. [sent-293, score-0.824]
97 This framework enabled us to uncover close connections between dropout training, adaptively balanced L2 -regularization, and AdaGrad; and led to a simple yet effective method for semi-supervised training. [sent-294, score-0.718]
98 There seem to be multiple opportunities for digging deeper into the connection between dropout training and adaptive regularization. [sent-295, score-0.857]
99 In particular, it would be interesting to see whether the dropout regularizer takes on a tractable and/or interpretable form in neural networks, and whether similar semi-supervised schemes could be used to improve on the results presented in [1]. [sent-296, score-0.792]
100 Text classification from labeled and unlabeled documents using EM. [sent-363, score-0.214]
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