nips nips2004 nips2004-119 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Philip M. Long, Xinyu Wu
Abstract: We establish a mistake bound for an ensemble method for classification based on maximizing the entropy of voting weights subject to margin constraints. The bound is the same as a general bound proved for the Weighted Majority Algorithm, and similar to bounds for other variants of Winnow. We prove a more refined bound that leads to a nearly optimal algorithm for learning disjunctions, again, based on the maximum entropy principle. We describe a simplification of the on-line maximum entropy method in which, after each iteration, the margin constraints are replaced with a single linear inequality. The simplified algorithm, which takes a similar form to Winnow, achieves the same mistake bounds. 1
Reference: text
sentIndex sentText sentNum sentScore
1 sg Abstract We establish a mistake bound for an ensemble method for classification based on maximizing the entropy of voting weights subject to margin constraints. [sent-7, score-0.624]
2 The bound is the same as a general bound proved for the Weighted Majority Algorithm, and similar to bounds for other variants of Winnow. [sent-8, score-0.194]
3 We prove a more refined bound that leads to a nearly optimal algorithm for learning disjunctions, again, based on the maximum entropy principle. [sent-9, score-0.366]
4 We describe a simplification of the on-line maximum entropy method in which, after each iteration, the margin constraints are replaced with a single linear inequality. [sent-10, score-0.415]
5 The simplified algorithm, which takes a similar form to Winnow, achieves the same mistake bounds. [sent-11, score-0.169]
6 1 Introduction In this paper, we analyze a maximum-entropy procedure for ensemble learning in the online learning model. [sent-12, score-0.058]
7 During the tth trial, the algorithm (1) receives xt ∈ {0, 1}n (interpreted in this work as a vector of base classifier predictions), (2) predicts a class yt ∈ {0, 1}, and (3) discovers the correct class yt . [sent-14, score-0.546]
8 During ˆ trial t, the algorithm has access only to information from previous trials. [sent-15, score-0.117]
9 The first algorithm we will analyze for this problem was proposed by Jaakkola, Meila and Jebara [14]. [sent-16, score-0.041]
10 The algorithm, at each trial t, makes its prediction by taking a weighted vote over the predictions of the base classifiers. [sent-17, score-0.158]
11 The weight vector pt is the probability distribution over the n base classifiers that maximizes the entropy, subject to the constraint that pt correctly classifies all patterns seen in previous trials with a given margin γ. [sent-18, score-1.863]
12 That is, it maximizes the entropy of pt subject to the constraints that pt · xs ≥ 1/2 + γ whenever ys = 1 for s < t, and pt · xs ≤ 1/2 − γ whenever ys = 0 for s < t. [sent-19, score-2.998]
13 We show that, if there is a weighting p∗ , determined with benefit of hindsight, that achieves margin γ on all trials, then this on-line maximum entropy procedure makes at most ln n 2γ 2 mistakes. [sent-20, score-0.487]
14 Littlestone [19] proved the same bound for the Weighted Majority Algorithm [21], and a similar bound for the Balanced Winnow Algorithm [19]. [sent-21, score-0.151]
15 The original Winnow algorithm was designed to solve the problem of learning a hidden disjunction of a small number k out of a possible n boolean variables. [sent-22, score-0.062]
16 When this problem is reduced to our general setting in the most natural way, the resulting bound is Θ(k 2 log n), whereas Littlestone proved a bound of ek ln n for Winnow. [sent-23, score-0.352]
17 We prove more refined bounds for a wider family of maximum-entropy algorithms, which use thresholds different than 1/2 (as proposed in [14]) and class-sensitive margins. [sent-24, score-0.058]
18 A mistake bound of ek ln n for learning disjunctions is a consequence of this more refined analysis. [sent-25, score-0.521]
19 The optimization needed at each round can be cast as minimizing a convex function subject to convex constraints, and thus can be solved in polynomial time [25]. [sent-26, score-0.155]
20 However, the same mistake bounds hold for a similar, albeit linear-time, algorithm. [sent-27, score-0.212]
21 This algorithm, after each trial, replaces all constraints from previous trials with a single linear inequality. [sent-28, score-0.097]
22 Littlestone [19] analyzed some variants of Winnow by showing that mistakes cause a reduction in the relative entropy between the learning algorithm’s weight vector, and that of the target function. [sent-31, score-0.322]
23 Kivinen and Warmuth [16] showed that an algorithm related to Winnow trades optimally in a sense between accommodating the information from new data, and keeping the relative entropy between the new and old weight vectors small. [sent-32, score-0.307]
24 As in related analyses like mistake bounds for the perceptron algorithm [22], Winnow [19] and the Weighted Majority Algorithm [19], our bound holds for any sequence of (xt , yt ) pairs satisfying the separation condition; in particular no independence assumptions are needed. [sent-36, score-0.428]
25 The proof of the main result proceeds roughly as follows. [sent-40, score-0.064]
26 If there is a mistake on trial t, it is corrected with a large margin by pt+1 . [sent-41, score-0.358]
27 Thus pt+1 must assign a significantly different probability to the voters predicting 1 on trial t than pt does. [sent-42, score-0.9]
28 Applying an identity known as Pinsker’s inequality, this means that the relative entropy from pt+1 and pt is large. [sent-43, score-1.038]
29 Next, we exploit the fact that the constraints satisfied by pt , and therefore by pt+1 , are convex to show that moving from pt to pt+1 must take you away from the uniform distribution, thus decreasing the entropy. [sent-44, score-1.662]
30 The theorem then follows from the fact that the entropy can only be reduced by a total of ln n. [sent-45, score-0.411]
31 The refinement leading to a ek ln n bound for disjunctions arises from the observation that Pinsker’s inequality can be strengthened when the probabilities being compared are small. [sent-46, score-0.374]
32 The analysis of this paper lends support to a view of Winnow as a fast, incremental approximation to the maximum entropy discrimination approach, and suggests a variant of Winnow that corresponds more closely to the inductive bias of maximum entropy. [sent-47, score-0.301]
33 For a feature vector x ∈ {0, 1}n and a class designation y ∈ {0, 1}, say that a probability distribution p is correct with margin γ if σ(p · x) = y, and |p · x − 1/2| ≥ γ. [sent-55, score-0.178]
34 If x and y were encountered in a trial of a learning algorithm, we say that p is correct with margin γ on that trial. [sent-56, score-0.229]
35 2 Entropy, relative entropy, and variation Recall that, for a probability distributions p = (p1 , . [sent-58, score-0.055]
36 (1) i=1 Relative entropy and variation distance are related in Pinsker’s inequality. [sent-65, score-0.26]
37 3 Information geometry Relative entropy obeys something like the Pythogarean Theorem. [sent-68, score-0.262]
38 Lemma 2 ([9]) Suppose q is a probability distribution, C is a convex set of probability distributions, and r is the element of A that minimizes D(r||q). [sent-69, score-0.086]
39 In our analysis, we will assume that there is always a feasible pt . [sent-73, score-0.789]
40 Theorem 3 If there is a fixed probability distribution p∗ that is correct with margin γ on all trials, OMEγ makes at most ln n mistakes. [sent-75, score-0.278]
41 2γ 2 Proof: We will show that a mistake causes the entropy of the hypothesis to drop by at least 2γ 2 . [sent-76, score-0.424]
42 Since the constraints only become more restrictive, the entropy never increases, and so the fact that the entropy lies between 0 and ln n will complete the proof. [sent-77, score-0.608]
43 The definition of pt+1 ensures that pt+1 · xt is on the correct side of 1/2 by at least γ. [sent-79, score-0.204]
44 Either pt+1 · xt − pt · xt ≥ γ, or the bitwise complement c(xt ) of xt satisfies pt+1 · c(xt ) − pt · c(xt ) ≥ γ. [sent-82, score-2.07]
45 Therefore, Pinsker’s Inequality (Lemma 1) implies that D(pt+1 ||pt ) ≥ 2γ 2 . [sent-84, score-0.037]
46 (2) Let Ct be the set of all probability distributions that satisfy the constraints in effect when pt was chosen, and let u = (1/n, . [sent-85, score-0.87]
47 Since pt+1 is in Ct (it must satisfy the constraints that pt did), Lemma 2 implies D(pt+1 ||u) ≥ D(pt+1 ||pt ) + D(pt ||u) and thus D(pt+1 ||u) − D(pt ||u) ≥ D(pt+1 ||pt ) which, since D(p||u) = (ln n) − H(p) for all p, implies H(pt )−H(pt+1 ) ≥ D(pt+1 ||pt ). [sent-89, score-0.929]
48 Because H(pt ) is always at least H(p∗ ), the same analysis leads to a mistake bound of (ln n − H(p∗ ))/(2γ 2 ). [sent-92, score-0.235]
49 Further, a nearly identical proof establishes the following (details are omitted from this abstract). [sent-93, score-0.048]
50 Theorem 4 Suppose OMEγ is modified so that p1 is set to be something other than the uniform distribution, and each pt minimizes D(pt ||p1 ) subject to the same constraints. [sent-94, score-0.877]
51 If there is a fixed p∗ that is correct with margin γ on all trials, the modified algorithm ∗ makes at most D(p ||p1 ) mistakes. [sent-95, score-0.173]
52 2γ 2 4 Maximum Entropy for Learning Disjunctions In this section, we show how the maximum entropy principle can be used to efficiently learn disjunctions. [sent-96, score-0.264]
53 For a feature vector x ∈ {0, 1}n and a class designation y ∈ {0, 1}, say that p is correct at threshold b with margin γ if σb (p · x) = y, and |p · x − b| ≥ γ. [sent-98, score-0.188]
54 Theorem 6 Suppose there is a probability distribution p∗ that is correct at threshold b, with a margin γ+ on all trials t with yt = 1, and with margin γ− on all trials with yt = 0. [sent-106, score-0.602]
55 Proof: The outline of the proof is similar to the proof of Theorem 3. [sent-108, score-0.096]
56 We will show that mistakes cause the entropy of the algorithm’s hypothesis to decrease. [sent-109, score-0.286]
57 Arguing as in the proof of Theorem 3, H(pt+1 ) ≤ H(pt ) − D(pt+1 ||pt ). [sent-110, score-0.048]
58 Lemma 5 then implies that H(pt+1 ) ≤ H(pt ) − d(pt+1 · xt ||pt · xt ). [sent-111, score-0.365]
59 (3) If there was a mistake on trial t for which yt = 1, then pt · xt ≤ b, and pt+1 · xt ≥ b + γ+ . [sent-112, score-1.496]
60 Thus in this case d(pt+1 · xt ||pt · xt ) ≥ d(b + γ+ ||b). [sent-113, score-0.328]
61 Similarly, if there was a mistake on trial t for which yt = 0, then d(pt+1 · xt ||pt · xt ) ≥ d(b − γ− ||b). [sent-114, score-0.707]
62 Once again, these two bounds on d(pt+1 · xt ||pt · xt ), together with (3) and the fact that the entropy is between 0 and ln n, complete the proof. [sent-115, score-0.709]
63 The analysis of Theorem 6 can also be used to prove bounds for the case in which mistakes of different types have different costs, as considered in [12]. [sent-116, score-0.089]
64 For example, if γ = 1/4, Theorem 6 gives a bound of 7. [sent-118, score-0.066]
65 Corollary 7 If there are k of the n features, such that each yt is the disjunction of those features in xt , then algorithm OME1/(ek),1/k−1/(ek),1/(ek) makes at most ek ln n mistakes. [sent-121, score-0.56]
66 Proof Sketch: If the target weight vector p∗ assigns equal weight to each of the variables in the disjunction, when y = 1, the weight of variables evaluating to 1 is at least 1/k, and when y = 0, it is 0. [sent-122, score-0.083]
67 Plugging into Theorem 6, simplifying and overapproximating completes the proof. [sent-124, score-0.041]
68 Note that in the case of disjunctions, it leads to a weaker 6k ln n bound. [sent-130, score-0.126]
69 Theorem 9 If there is a probability distribution p∗ that is correct at threshold b with a margin γ on all trials, then OMEb,γ,γ makes at most 3bγln n mistakes. [sent-131, score-0.192]
70 2 5 Relaxed on-line maximum entropy algorithms Let us refer the halfspace of probability distributions that satisfy the constraint of trial t as Tt and the associated separating hyperplane by Jt . [sent-132, score-0.536]
71 Recall that Ct is the set of feasible Figure 1: In ROME, the constraints Ct in effect before the tth round are replaced by the halfspace St . [sent-133, score-0.178]
72 solutions to all the constraints in effect when pt is chosen. [sent-134, score-0.832]
73 So pt+1 maximizes entropy subject to membership in Ct+1 = Tt ∩ Ct . [sent-135, score-0.351]
74 Our proofs only used the following facts about the OME algorithm: (a) pt+1 ∈ Tt , (b) pt is the maximum entropy member of Ct , and (c) pt+1 ∈ Ct . [sent-136, score-1.121]
75 Suppose At is the set of weight vectors with entropy at last that of pt . [sent-137, score-1.038]
76 Finally, let St be the halfspace with boundary Ht containing pt+1 . [sent-139, score-0.051]
77 (The least obvious is (b), which follows since Ht is tangent to At at pt , and the entropy function is strictly concave. [sent-142, score-1.036]
78 ) Also, as previously observed by Littlestone [19], the algorithm might just as well not respond to trials in which there is not a mistake. [sent-143, score-0.075]
79 Proposition 10 If trial t is a mistake, and q maximizes entropy subject to membership in St ∩ Tt , then it is on the separating hyperplane for Tt . [sent-148, score-0.534]
80 Proof: Because q and p both satisfy St , any convex combination of the two satisfies St . [sent-149, score-0.064]
81 Thus, if q was on the interior of Tt , we could find a probability distribution with higher entropy that still satisfies both St and Tt by taking a tiny step from q toward p. [sent-150, score-0.242]
82 This would contradict the assumption that q is the maximum entropy member of St ∩ Tt . [sent-151, score-0.301]
83 This implies that the next hypothesis of a ROME algorithm is either on Jt (the separating hyperplane Tt ) only, or on both Jt and Ht (the separating hyperplane of St ). [sent-152, score-0.26]
84 The following theorem will enable us to obtain a formula in either case. [sent-153, score-0.073]
85 1)) Suppose q is a probability distribution, and C is a set defined by linear constraints as follows: for an m × n real matrix A, and a m-dimensional column vector b, C = {r : Ar = b}. [sent-155, score-0.058]
86 Then if r is the member of C minimizing D(r||q), then there are scalar constants Z, c1 , . [sent-156, score-0.037]
87 If the next hypothesis pt+1 of a ROME algorithm is on Ht , then by Lemma 2, it and all other members of Ht satisfy D(pt+1 ||u) = D(pt+1 ||pt ) + D(pt ||u). [sent-163, score-0.09]
88 Thus, in this case, pt+1 also minimizes D(q||pt ) from among the members q of Ht ∩ Jt . [sent-164, score-0.033]
89 Thus, Lemma 11 implies that pt+1,i /pt,i is the same for all i with xi = 1, and the same for all i with xi = 0. [sent-165, score-0.037]
90 This implies that, for ROMEb,γ+ ,γ− , if there was a mistake on a trial t, (b+γ )p + t,i if xt,i = 1 and yt = 1 pt ·xt (1−(b+γ ))p + t,i if xt,i = 0 and yt = 1 1−(pt ·xt ) pt+1,i = (4) (b−γ− )pt,i if xt,i = 1 and yt = 0 pt ·xt (1−(b−γ+ ))pt,i if x = 0 and y = 0. [sent-166, score-2.222]
91 If pt+1 is not on the separating hyperplane for St , then it must maximize entropy subject to membership in Tt alone, and therefore subject to membership in Jt . [sent-168, score-0.534]
92 In this case, Lemma 11 implies (b+γ ) if xt,i = 1 and yt = 1 |{j:xt,j+ =1}| (1−(b+γ )) + if xt,i = 0 and yt = 1 |{j:xt,j =0}|. [sent-169, score-0.265]
93 pt+1,i = (5) (b−γ+ ) |{j:x =1}| if xt,i = 1 and yt = 0 t,j (1−(b−γ+ )) if xt,i = 0 and yt = 0 |{j:xt,j =0}|. [sent-170, score-0.228]
94 If this is the case, then pt+1 defined as in (5) should be a member of St . [sent-171, score-0.037]
95 Evaluating the gradient of H at pt , and simplifying a bit, we can see that n 1 St = q : qi ln ≤ H(p) . [sent-173, score-0.957]
96 pt,i i=1 Summing up, a way to implement a ROME algorithm with the same mistake bound as the corresponding OME algorithm is to • try defining pt+1 as in (5), and check whether the resulting pt+1 ∈ St , if so use it, and • if not, then define pt+1 as in (4) instead. [sent-174, score-0.277]
97 Acknowledgements We are grateful to Tony Jebara and Tong Zhang for helpful conversations, and an anonymous referee for suggesting a simplification of the proof of Theorem 3. [sent-175, score-0.048]
98 An analog of the minimax theorem for vector payoffs. [sent-191, score-0.073]
99 A new algorithm for minimizing convex functions over convex sets. [sent-324, score-0.103]
100 A sequential approximation bound for some sample-dependent convex optimization problems with applications in learning. [sent-332, score-0.107]
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