nips nips2009 nips2009-63 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Peilin Zhao, Steven C. Hoi, Rong Jin
Abstract: In most online learning algorithms, the weights assigned to the misclassified examples (or support vectors) remain unchanged during the entire learning process. This is clearly insufficient since when a new misclassified example is added to the pool of support vectors, we generally expect it to affect the weights for the existing support vectors. In this paper, we propose a new online learning method, termed Double Updating Online Learning, or DUOL for short. Instead of only assigning a fixed weight to the misclassified example received in current trial, the proposed online learning algorithm also tries to update the weight for one of the existing support vectors. We show that the mistake bound can be significantly improved by the proposed online learning method. Encouraging experimental results show that the proposed technique is in general considerably more effective than the state-of-the-art online learning algorithms. 1
Reference: text
sentIndex sentText sentNum sentScore
1 edu Abstract In most online learning algorithms, the weights assigned to the misclassified examples (or support vectors) remain unchanged during the entire learning process. [sent-19, score-0.384]
2 This is clearly insufficient since when a new misclassified example is added to the pool of support vectors, we generally expect it to affect the weights for the existing support vectors. [sent-20, score-0.261]
3 In this paper, we propose a new online learning method, termed Double Updating Online Learning, or DUOL for short. [sent-21, score-0.239]
4 Instead of only assigning a fixed weight to the misclassified example received in current trial, the proposed online learning algorithm also tries to update the weight for one of the existing support vectors. [sent-22, score-0.483]
5 We show that the mistake bound can be significantly improved by the proposed online learning method. [sent-23, score-0.378]
6 Encouraging experimental results show that the proposed technique is in general considerably more effective than the state-of-the-art online learning algorithms. [sent-24, score-0.27]
7 Most online learning algorithms work by assigning a fixed weight to a new example when it is misclassified. [sent-28, score-0.303]
8 This is clearly insufficient because when a new example is added to the pool of support vectors, we expect it to affect the weights assigned to the existing support vectors received in previous trials. [sent-30, score-0.364]
9 Although several online algorithms are capable of updating the example weights as the learning process goes, most of them are designed for the purposes other than improving the classification accuracy and reducing the mistake bound. [sent-31, score-0.507]
10 , 2005), online learning algorithms are proposed to adjust the example weights in order to fit in the constraint of fixed number of support vectors; in (Cesa-Bianchi & Gentile, 2006), example weights are adjusted to track the drifting concepts. [sent-35, score-0.469]
11 In this paper, we propose a new formulation for online learning that aims to dynamically update the example weights in order to improve the classification accuracy as well as the mistake bound. [sent-36, score-0.428]
12 Instead of only assigning a weight to the misclassified example that is received in current trial, the proposed online learning algorithm also updates the weight for one of the existing support vectors. [sent-37, score-0.481]
13 The key question in the proposed online learning approach is which one of the existing support vectors should be selected for weight updating. [sent-40, score-0.465]
14 To this end, we employ an analysis for double updating online learning that is based on the recent work of online convex programming by incremental dual ascent (Shalev-Shwartz & Singer, 2006). [sent-41, score-0.685]
15 Our analysis shows that under certain conditions, the proposed online learning algorithm can significantly reduce the mistake bound of the existing online algorithms. [sent-42, score-0.635]
16 This result is further verified empirically by extensive experiments and comparison to the state-of-the-art algorithms for online learning. [sent-43, score-0.267]
17 Section 2 reviews the related work for online learning. [sent-45, score-0.239]
18 Section 3 presents the proposed “double updating” approach to online learning. [sent-46, score-0.252]
19 One of the most well-known online approaches is the Perceptron algorithm (Rosenblatt, 1958; Freund & Schapire, 1999), which updates the learning function by adding a new example with a constant weight into the current set of support vectors when it is misclassified. [sent-54, score-0.452]
20 Recently a number of online learning algorithms have been developed based on the criterion of maximum margin (Crammer & Singer, 2003; Gentile, 2001; Kivinen et al. [sent-55, score-0.314]
21 Empirical studies showed that the maximum margin based online learning algorithms are generally more effective than the Perceptron algorithm. [sent-62, score-0.287]
22 However, despite the difference, most online learning algorithms only update the weight of the newly added support vector, and keep the weights of the existing support vectors unchanged. [sent-63, score-0.663]
23 This constraint could significantly limit the effect of online learning. [sent-64, score-0.239]
24 Besides the studies for regular online learning, several algorithms are proposed for online learning with fixed budget. [sent-65, score-0.519]
25 In these studies, the total number of support vectors is required to be bounded either by a theoretical bound or by a manually fixed budget. [sent-66, score-0.187]
26 Example algorithms for fixed budget online learning include (Weston & Bordes, 2005; Crammer et al. [sent-67, score-0.323]
27 The key idea of these algorithms is to dynamically update the weights of the existing support vectors as a new support vector is added, and the support vector with the least weight will be discarded when the number of support vectors exceeds the budget. [sent-71, score-0.669]
28 The idea of discarding support vectors is also used in studies (Kivinen et al. [sent-72, score-0.186]
29 , 2008), a new “projection” approach is proposed for online learning that ensures the number of support vectors is bounded. [sent-76, score-0.411]
30 Besides, in (Cesa-Bianchi & Gentile, 2006), an online learning algorithm is proposed to handle the drifting concept, in which the weights of the existing support vectors are reduced whenever a new support vector is added. [sent-77, score-0.584]
31 Although these online learning algorithms are capable of dynamically adjusting the weights of support vectors, they are designed to either fit in the budget of the number of support vectors or to handle drifting concepts, not to improve the classification accuracy and the mistake bound. [sent-78, score-0.762]
32 The proposed online learning algorithm is closely related to the recent work of online convex programming by incremental dual ascent (Shalev-Shwartz & Singer, 2006). [sent-79, score-0.523]
33 Although the idea of simultaneously updating the weights of multiple support vectors was mentioned in (Shalev-Shwartz & Singer, 2006), no efficient updating algorithm was explicitly proposed. [sent-80, score-0.381]
34 As will be shown later, the online algorithm proposed in this work shares the same computational cost as that of conventional online learning algorithms, despite the need of updating weights of two support vectors. [sent-81, score-0.753]
35 1 Motivation We consider an online learning trial t with an incoming example that is misclassified. [sent-83, score-0.262]
36 , αn ) ∈ [0, C]n the weights assigned to the support vectors in D, where C is a predefined constant. [sent-93, score-0.208]
37 The resulting classifier, denoted by f (x), is expressed as n f (x) = αi yi κ(x, xi ) (1) i=1 Let (xa , ya ) be the misclassified example received in the trial t, i. [sent-94, score-0.405]
38 Let (xa , ya ) be an example misclassified by the current classifier f (x) = n i=1 αi yi κ(x, xi ), i. [sent-99, score-0.357]
39 Let f (x) = βya κ(x, xa ) + f (x) be the updated classifier with β > 0. [sent-102, score-0.213]
40 There exists at least one support vector xi ∈ D such that yi f (xi ) > yi f (xi ). [sent-103, score-0.268]
41 It follows from the fact that: ∃xi ∈ D, yi ya κ(xi , xa ) < 0 when ya f (xa ) < 0. [sent-105, score-0.764]
42 In the case when ya f (xa ) ≤ −γ, it is easy to verify that there exists some support vector (xb , yb ) who satisfies βya yb k(xa , xb ) ≤ −γ/n; at the meantime, it can be shown that when the classification confidence of (xb , yb ) is less than γ/n, i. [sent-107, score-1.565]
43 , yb f (xb ) ≤ γ/n, such support vector will be misclassified after the classifier is updated with the example (xa , ya ). [sent-109, score-0.676]
44 In order to alleviate this problem, we propose to update the weight for the existing support vector whose classification confidence is significantly affected by the new misclassified example. [sent-110, score-0.17]
45 In particular, we consider a support vector (xb , yb ) ∈ D for weight updating if it satisfies the following two conditions • yb f (xb ) ≤ 0, i. [sent-111, score-0.858]
46 , support vector (xb , yb ) is misclassified by the current classifier f (x) • k(xb , xa )ya yb ≤ −ρ where ρ ≥ 0 is a predefined threshold, i. [sent-113, score-0.927]
47 , support vector (xb , yb ) “conflicts” with the new misclassified example (xa , ya ). [sent-115, score-0.662]
48 We refer to the support vector satisfying the above conditions as auxiliary example. [sent-116, score-0.158]
49 It is clear that by adding the misclassified example (xa , ya ) to classifier f (x) with weight β, the classification score of (xb , yb ) will be reduced by at least βρ, which could lead to the misclassification of the auxiliary example (xb , yb ). [sent-117, score-0.995]
50 To avoid such a mistake, we propose to update the weights for both (xa , ya ) and (xb , yb ) simultaneously. [sent-118, score-0.62]
51 In the next section, we show the details of the double updating algorithm for online learning, and the analysis for mistake bound. [sent-119, score-0.526]
52 Our analysis follows closely the previous work on the relationship between online learning and the dual formulation of SVM (Shalev-Shwartz & Singer, 2006), in which the online learning is interpreted as an efficient updating rule for maximizing the objective function in the dual form of SVM. [sent-120, score-0.636]
53 If an online learning algorithm A is designed to ensure that all ∆t is bounded from the below by a positive constant ∆, then the number of mistakes made by A when trained over a sequence of trials (x1 , y1 ), . [sent-122, score-0.35]
54 In our analysis, we will show that ∆, which is referred to as the bounding constant for the improvement in the objective function, could be significantly improved when updating the weight for both the newly misclassified example and the auxiliary example. [sent-126, score-0.208]
55 For the remaining part of this paper, we denote by (xb , yb ) an auxiliary example that satisfies the two conditions specified before. [sent-127, score-0.378]
56 , αn−1 )) ∈ Rn−1 to denote the weights assigned to all the support vectors in D except (xb , yb ). [sent-131, score-0.524]
57 , yn−1 ) ∈ [−1, 1]n−1 the class labels assigned to all the examples in D except for (xb , yb ). [sent-135, score-0.331]
58 We define sa = κ(xa , xa ), sb = κ(xb , xb ), sab = κ(xa , xb ), wab = ya yb sab . [sent-136, score-1.503]
59 (4) According to the assumption of auxiliary example, we have wab = sab ya yb ≤ −ρ. [sent-137, score-0.725]
60 Finally, we denote by γb the weight for the auxiliary example (xb , yb ) that is used in the current classifier f (x), and by γa and γb the updated weights for (xa , ya ) and (xb , yb ), respectively. [sent-138, score-1.028]
61 2 Double Updating Online Learning Recall an auxiliary example (xb , yb ) should satisfy two conditions (I) yb f (xb ) ≤ 0, and (II) wab ≤ −ρ. [sent-141, score-0.759]
62 In addition, the new example (xa , ya ) received in the current iteration t is misclassified, i. [sent-142, score-0.275]
63 Following the framework of dual formulation for online learning, the following lemma shows how to compute ∆t , i. [sent-145, score-0.271]
64 , the improvement in the objective function of dual SVM by adjusting weights for (xa , ya ) and (xb , yb ). [sent-147, score-0.649]
65 It is straightforward to verify that the dual function of min ft 2 Hκ +C t i=1 (6) (yi ft (xi )), denoted by Dt (γ1 , . [sent-150, score-0.499]
66 , γt ) = t γi − i=1 γi yi ft (xi ) + i=1 t i=1 1 ft 2 2 Hκ (7) where 0 ≤ γi ≤ C, i = 1, . [sent-156, score-0.519]
67 , t and ft (·) = γi yi κ(·, xi ) is the current classifier. [sent-159, score-0.334]
68 Note that this constraint does not come directly from the box constraint that the weight for example (xb , yb ) is in the range [0, C], i. [sent-173, score-0.352]
69 , sb 2 g(∆γb ) = ∆γb (1 − yb ft−1 (xb ) − wab γa ) − ∆γb 2 Since wab ≤ −ρ and yb ft−1 (xb ) ≤ 0, it is clear that ∆γb ≥ 0 when maximizing g(∆γb ), which results in the constraint ∆γb ≥ 0. [sent-178, score-0.802]
70 Assume C > γb + 1/(1 − ρ) for the selected auxiliary example (xb , yb ). [sent-181, score-0.378]
71 We have the following bound for ∆, when updating the weights for the new example (xa , ya ) and the auxiliary example (xb , yb ) ∆≥ 1 1 + min (1 + ρ)2 , (C − γ)2 2 2 Proof. [sent-188, score-0.783]
72 The final remaining question is how to identify the auxiliary example (xb , yb ) efficiently, which requires efficiently updating the classification score yi f (xi ) for all the support vectors. [sent-191, score-0.648]
73 This updating procedure ensures that the computational cost of double updating online learning is O(n), where n is the number of support vectors, similar to that of the kernel online learning algorithm. [sent-194, score-0.867]
74 We denote by Ms the number of mistakes when we made a single update without finding appropriate auxiliary example. [sent-205, score-0.179]
75 1 Experimental Testbed and Setup We now evaluate the empirical performance of the proposed double updating online learning (DUOL) algorithm. [sent-214, score-0.427]
76 Finally, as an ideal yardstick, we also implement a full online SVM algorithm (“Online-SVM”) (Shalev-Shwartz & Singer, 2006), which updates all the support vectors in each trial, and is thus computationally extremely intensive as will be revealed in our study. [sent-222, score-0.416]
77 We evaluate the online learning performance by measuring mistake rate, i. [sent-234, score-0.351]
78 , the ratio of the number of mistakes made by the online learning algorithm over the total number of examples received for predictions. [sent-236, score-0.361]
79 In addition, to examine the sparsity of the resulting classifiers, we also evaluate the number of support vectors produced by each online learning algorithm. [sent-237, score-0.398]
80 Figure 2 to 6 show the mistake rates of all online learning algorithms in comparison over trials. [sent-243, score-0.379]
81 We observe that Online-SVM yields considerably better performance than the other online learning algorithms for dataset “german”, “splice”, “spambase”, and “MITFace”, however, at the price of extremely high computational cost. [sent-244, score-0.285]
82 For most cases, the running time of Online-SVM is two order, sometimes three order, higher than the other online learning algorithms, making it 1 http://www. [sent-245, score-0.239]
83 For the remaining part of this section, we restrict our discussion to the other six baseline online learning algorithms. [sent-257, score-0.258]
84 We also notice that the agg-ROMMA and the two PA algorithms consume considerably larger numbers of support vectors than the other three algorithms. [sent-261, score-0.205]
85 Second, comparing with all six competing algorithms, we observe that DUOL achieves significantly smaller mistake rates than the other single-updating algorithms in all cases. [sent-264, score-0.159]
86 This shows that the proposed double updating approach is effective in improving the online prediction performance. [sent-265, score-0.427]
87 By examining the sparsity of resulting classifiers, we observed that DUOL results in sparser classifiers than the three aggressive online learning algorithms, and denser classifiers than the three non-aggressive algorithms. [sent-266, score-0.266]
88 Third, according to the results of running time, we observe that DUOL is overall efficient compared to the state-of-the-art online learning algorithms. [sent-267, score-0.239]
89 677 Conclusions This paper presented a novel “double updating” approach to online learning named as “DUOL”, which not only updates the weight of the newly added support vector, but also adjusts the weight of one existing support vector that seriously conflicts with the new support vector. [sent-512, score-0.668]
90 We show that the mistake bound for an online classification task can be significantly reduced by the proposed DUOL algorithms. [sent-513, score-0.378]
91 Future work will address issues of multi-class double updating online learning. [sent-516, score-0.414]
92 4 PA−I PA−II Online−SVM DUOL Online average number of support vectors Online average rate of mistakes 0. [sent-519, score-0.321]
93 25 0 200 400 600 Number of samples 800 0 1000 (a) average rate of mistakes 0 200 400 600 Number of samples 800 1000 0 (b) average number of support vectors 200 400 600 Number of samples 800 1000 (c) average time cost (log10 t) Figure 2: Evaluation on the german dataset. [sent-527, score-0.455]
94 9) 3 2 average time cost (log10 t) Online average rate of mistakes Online average number of support vectors Perceptron ROMMA agg−ROMMA ALMA (0. [sent-537, score-0.37]
95 9) 7000 average time cost (log10 t) Online average rate of mistakes Online average number of support vectors Perceptron ROMMA agg−ROMMA ALMA (0. [sent-568, score-0.37]
96 27 0 2000 4000 6000 8000 10000 Number of samples 12000 14000 −2 16000 (b) average number of support vectors 0 2000 4000 6000 8000 10000 Number of samples 12000 14000 16000 (c) average time cost (log10 t) Figure 5: Evaluation on the a7a dataset. [sent-570, score-0.273]
97 3500 Online average rate of mistakes Online average number of support vectors Perceptron ROMMA agg−ROMMA ALMA (0. [sent-572, score-0.321]
98 5 Number of samples 2 (a) average rate of mistakes 2. [sent-591, score-0.157]
99 5 4 x 10 (b) average number of support vectors −2 0 0. [sent-595, score-0.184]
100 The forgetron: A kernel-based perceptron on a fixed budget. [sent-655, score-0.198]
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