# The Optimality of Naive Bayes

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1 The Optimality of Naive Bayes Harry Zhang Faculty of Computer Science University of New Brunswick Fredericton, New Brunswick, Canada E3B 5A3 Abstract Naive Bayes is one of the most efficient and effective inductive learning algorithms for machine learning and data mining Its competitive performance in classification is surprising, because the conditional independence assumption on which it is based, is rarely true in realworld applications An open question is: what is the true reason for the surprisingly good performance of naive Bayes in classification? In this paper, we propose a novel explanation on the superb classification performance of naive Bayes We show that, essentially, the dependence distribution; ie, how the local dependence of a node distributes in each class, evenly or unevenly, and how the local dependencies of all nodes work together, consistently (supporting a certain classification) or inconsistently (canceling each other out), plays a crucial role Therefore, no matter how strong the dependences among attributes are, naive Bayes can still be optimal if the dependences distribute evenly in classes, or if the dependences cancel each other out We propose and prove a sufficient and necessary conditions for the optimality of naive Bayes Further, we investigate the optimality of naive Bayes under the Gaussian distribution We present and prove a sufficient condition for the optimality of naive Bayes, in which the dependence between attributes do exist This provides evidence that dependence among attributes may cancel out each other In addition, we explore when naive Bayes works well Rule, the probability of an example E =(x,x 2,,x n ) being class c is p(c E) = p(e c)p(c) p(e) E is classified as the class C =+if and only if p(c =+ E) f b (E) =, () p(c = E) where f b (E) is called a Bayesian classifier Assume that all attributes are independent given the value of the class variable; that is, p(e c) =p(x,x 2,,x n c) = p(x i c), the resulting classifier is then: f nb (E) = p(c =+) p(c = ) p(x i C =+) p(x i C = ) (2) The function f nb (E) is called a naive Bayesian classifier, or simply naive Bayes (NB) Figure shows an example of naive Bayes In naive Bayes, each attribute node has no parent except the class node Naive Bayes and Augmented Naive Bayes Classification is a fundamental issue in machine learning and data mining In classification, the goal of a learning algorithm is to construct a classifier given a set of training examples with class labels Typically, an example E is represented by a tuple of attribute values (x,x 2,,,x n ), where x i is the value of attribute X i Let C represent the classification variable, and let c be the value of C In this paper, we assume that there are only two classes: + (the positive class) or (the negative class) A classifier is a function that assigns a class label to an example From the probability perspective, according to Bayes Copyright c 2004, American Association for Artificial Intelligence (wwwaaaiorg) All rights reserved C A A A A Figure : An example of naive Bayes Naive Bayes is the simplest form of Bayesian network, in which all attributes are independent given the value of the class variable This is called conditional independence It is obvious that the conditional independence assumption is rarely true in most real-world applications A straightforward approach to overcome the limitation of naive Bayes is

2 to extend its structure to represent explicitly the dependencies among attributes An augmented naive Bayesian network, or simply augmented naive Bayes (ANB), is an extended naive Bayes, in which the class node directly points to all attribute nodes, and there exist links among attribute nodes Figure 2 shows an example of ANB From the view of probability, an ANB G represents a joint probability distribution represented below p G (x,,x n,c)=p(c) p(x i pa(x i ),c), (3) where pa(x i ) denotes an assignment to values of the parents of X i We use pa(x i ) to denote the parents of X i ANB is a special form of Bayesian networks in which no node is specified as a class node It has been shown that any Bayesian network can be represented by an ANB (Zhang & Ling 200) Therefore, any joint probability distribution can be represented by an ANB A C A2 A3 A4 Figure 2: An example of ANB When we apply a logarithm to f b (E) in Equation, the resulting classifier log f b (E) is the same as f b (E), in the sense that an example E belongs to the positive class, if and only if log f b (E) 0 f nb in Equation 2 is similar In this paper, we assume that, given a classifier f, an example E belongs to the positive class, if and only if f(e) 0 Related Work Many empirical comparisons between naive Bayes and modern decision tree algorithms such as C45 (Quinlan 993) showed that naive Bayes predicts equally well as C45 (Langley, Iba, & Thomas 992; Kononenko 990; Pazzani 996) The good performance of naive Bayes is surprising because it makes an assumption that is almost always violated in realworld applications: given the class value, all attributes are independent An open question is what is the true reason for the surprisingly good performance of naive Bayes on most classification tasks? Intuitively, since the conditional independence assumption that it is based on is almost never hold, its performance may be poor It has been observed that, however, its classification accuracy does not depend on the dependencies; ie, naive Bayes may still have high accuracy on the datasets in which strong dependencies exist among attributes (Domingos & Pazzani 997) Domingos and Pazzani (Domingos & Pazzani 997) present an explanation that naive Bayes owes its good performance to the zero-one loss function This function defines the error as the number of incorrect classifications (Friedman 996) Unlike other loss functions, such as the squared error, the zero-one loss function does not penalize inaccurate probability estimation as long as the maximum probability is assigned to the correct class This means that naive Bayes may change the posterior probabilities of each class, but the class with the maximum posterior probability is often unchanged Thus, the classification is still correct, although the probability estimation is poor For example, let us assume that the true probabilities p(+ E) and p( E) are 09 and 0 respectively, and that the probability estimates p (+ E) and p ( E) produced by naive Bayes are 06 and 04 Obviously, the probability estimates are poor, but the classification (positive) is not affected Domingos and Pazzani s explanation (Domingos & Pazzani 997) is verified by the work of Frank et al (Frank et al 2000), which shows that the performance of naive Bayes is much worse when it is used for regression (predicting a continuous value) Moreover, evidence has been found that naive Bayes produces poor probability estimates (Bennett 2000; Monti & Cooper 999) In our opinion, however, Domingos and Pazzani (Domingos & Pazzani 997) s explanation is still superficial as it does not uncover why the strong dependencies among attributes could not flip the classification For the example above, why the dependencies could not make the probability estimates p (+ E) and p ( E) produced by naive Bayes be 04 and 06? The key point here is that we need to know how the dependencies affect the classification, and under what conditions the dependencies do not affect the classification There has been some work to explore the optimality of naive Bayes (Rachlin, Kasif, & Aha 994; Garg & Roth 200; Roth 999; Hand & Yu 200), but none of them give an explicit condition for the optimality of naive Bayes In this paper, we propose a new explanation that the classification of naive Bayes is essentially affected by the dependence distribution, instead by the dependencies among attributes In addition, we present a sufficient condition for the optimality of naive Bayes under the Gaussian distribution, and show theoretically when naive Bayes works well A New Explanation on the Superb Classification Performance of Naive Bayes In this section, we propose a new explanation for the surprisingly good classification performance of naive Bayes The basic idea comes from the observation as follows In a given dataset, two attributes may depend on each other, but the dependence may distribute evenly in each class Clearly, in this case, the conditional independence assumption is violated, but naive Bayes is still the optimal classifier Further, what eventually affects the classification is the combination of dependencies among all attributes If we just look at two attributes, there may exist strong dependence between them that affects the classification When the dependencies among all attributes work together, however, they

3 may cancel each other out and no longer affect the classification Therefore, we argue that it is the distribution of dependencies among all attributes over classes that affects the classification of naive Bayes, not merely the dependencies themselves Before discussing the details, we introduce the formal definition of the equivalence of two classifiers under zeroone loss, which is used as a basic concept Definition Given an example E, two classifiers f and f 2 are said to be equal under zero-one loss on E,iff (E) 0 if and only if f 2 (E) 0, denoted by f (E) = f 2 (E) If for every example E in the example space, f (E) = f 2 (E), f and f 2 are said to be equal under zero-one loss, denoted by f = f2 Local Dependence Distribution As discussed in the section above, ANB can represent any joint probability distribution Thus we choose an ANB as the underlying probability distribution Our motivation is to find out under what conditions naive Bayes classifies exactly the same as the underlying ANB Assume that the underlying probability distribution is an ANB G with two classes {+, }, and the dependencies among attributes are represented by the arcs among attribute nodes For each node, the influence of its parents is quantified by the correspondent conditional probabilities We call the dependence between a node and its parents local dependence of this node How do we measure the local dependence of a node in each class? Naturally, the ratio of the conditional probability of the node given its parents over the conditional probability of the node without the parents, reflects how strong the parents affect the node in each class Thus we have the following definition Definition 2 For a node X on ANB G, the local dependence derivative of X in classes + and are defined as below dd + p(x pa(x), +) G (x pa(x)) = (4) p(x +) dd p(x pa(x), ) G (x pa(x)) = (5) p(x ) Essentially, dd + G (x pa(x)) reflects the strength of the local dependence of node X in class +, which measures the influence of X s local dependence on the classification in class + dd G (x pa(x)) is similar Further, we have the following results When X has no parent, then dd + G (x pa(x)) = dd G (x pa(x)) = 2 When dd + G (x pa(x)), X s local dependence in class + supports the classification of C = + Otherwise, it supports the classification of C = Similarly, when dd G (x pa(x)), A s local dependence in class supports the classification of C = Otherwise, it supports the classification of C =+ Intuitively, when the local dependence derivatives in both classes support the different classifications, the local dependencies in the two classes cancel partially each other out, and the final classification that the local dependence supports, is the class with the greater local dependence derivative Another case is that the local dependence derivatives in the two classes support the same classification Then, the local dependencies in the two classes work together to support the classification The discussion above shows that the ratio of the local dependence derivatives in both classes ultimately determines which classification the local dependence of a node supports Thus we have the following definition Definition 3 For a node X on ANB G, the local dependence derivative ratio at node X, denoted by ddr G (x) is defined below: ddr G (x) = dd+ G (x pa(x)) dd (6) G (x pa(x)) From the above definition, ddr G (x) quantifies the influence of X s local dependence on the classification Further, we have the following results If X has no parents, ddr G (x) = 2 If dd + G (x pa(x)) = dd G (x pa(x)), ddr G(x) = This means that x s local dependence distributes evenly in class + and class Thus, the dependence does not affect the classification, no matter how strong the dependence is 3 If ddr G (x) >, X s local dependence in class + is stronger than that in class ddr G (x) < means the opposite Global Dependence Distribution Now let us explore under what condition an ANB works exactly the same as its correspondent naive Bayes The following theorem establishes the relation of an ANB and its correspondent naive Bayes Theorem Given an ANB G and its correspondent naive Bayes G nb (ie, remove all the arcs among attribute nodes from G) on attributes X, X 2,, X n, assume that f b and f nb are the classifiers corresponding to G and G nb, respectively For a given example E =(x, x 2,, x n ), the equation below is true f b (x,x 2,, x n )=f nb (x,x 2,, x n ) ddr G (x i ), (7) where n ddr G(x i ) is called the dependence distribution factor at example E, denoted by DF G (E) Proof: According to Equation 3, we have: f b (x,,x n)= p(+) p( ) = p(+) p( ) p(x i +) p(x i ) p(x i pa(x i), +) p(x i pa(x i), ) p(x i pa(x i), +)p(x i ) p(x i pa(x i), )p(x i +)

4 ddr + G = f nb (E) (xi pa(xi)) ddr G (xi pa(xi)) = f nb (E) ddr G(x i) = DF G(E)f nb (E) From Theorem, we know that, in fact, it is the dependence distribution factor DF G (E) that determines the difference between an ANB and its correspondent naive Bayes in the classification Further, DF G (E) is the product of local dependence derivative ratios of all nodes Therefore, it reflects the global dependence distribution (how each local dependence distributes in each class, and how all local dependencies work together) For example, when DF G (E) =, G has the same classification as G nb on E In fact, it is not necessary to require DF G (E) =, in order to make an ANB G has the same classification as its correspondent naive Bayes G nb, as shown in the theorem below Theorem 2 Given an example E = (x,x 2,, x n ), an ANB G is equal to its correspondent naive Bayes G nb under zero-one loss; ie, f b (E) = f nb (E) (Definition ), if and only if when f b (E), DF G (E) f b (E); or when f b (E) <, DF G (E) >f b (E) Proof: The proof is straightforward by apply Definition and Theorem From Theorem 2, if the distribution of the dependences among attributes satisfies certain conditions, then naive Bayes classifies exactly the same as the underlying ANB, even though there may exist strong dependencies among attributes Moreover, we have the following results: When DF G (E) =, the dependencies in ANB G has no influence on the classification That is, the classification of G is exactly the same as that of its correspondent naive Bayes G nb There exist three cases for DF G (E) = no dependence exists among attributes for each attribute X on G, ddr G (x) =; that is, the local distribution of each node distributes evenly in both classes the influence that some local dependencies support classifying E into C =+is canceled out by the influence that other local dependences support classifying E into C = 2 f b (E) = f nb (E) does not require that DF G (E) = The precise condition is given by Theorem 2 That explains why naive Bayes still produces accurate classification even in the datasets with strong dependencies among attributes (Domingos & Pazzani 997) 3 The dependencies in an ANB flip (change) the classification of its correspondent naive Bayes, only if the condition given by Theorem 2 is no longer true (8) Theorem 2 represents a sufficient and necessary condition for the optimality of naive Bayes on an example E If for each example E in the example space, f b (E) = f nb (E); ie, f b = fnb, then naive Bayes is globally optimal Conditions for the Optimality of Naive Bayes In Section, we proposed that naive Bayes is optimal if the dependences among attributes cancel each other out That is, under circumstance, naive Bayes is still optimal even though the dependences do exist In this section, we investigate naive Bayes under the multivariate Gaussian distribution and prove a sufficient condition for the optimality of naive Bayes, assuming the dependences among attributes do exist That provides us with theoretic evidence that the dependences among attributes may cancel each other out Let us restrict our discussion to two attributes X and X 2, and assume that the class density is a multivariate Gaussian in both the positive and negative classes That is, p(x,x 2, +) = p(x,x 2, ) = 2π e 2 (x µ + ) T + (x µ+ ), + /2 2π e 2 (x µ ) T (x µ ), /2 where x =(x,x 2 ), + and are the covariance matrices in the positive and negative classes respectively, and + are the determinants of and +, + and are the inverses of and + ; µ+ =(µ +,µ+ 2 ) and µ =(µ,µ 2 ), µ+ i and µ i are the means of attribute X i in the positive and negative classes respectively, and (x µ + ) T and (x µ ) T are the transposes of (x µ + ) and (x µ ) We assume that two classes have a common covariance matrix + = =, and X and X 2 have the same variance σ in both classes Then, when applying a logarithm to the Bayesian classifier, defined in Equation, we obtain the classifier f b below f b (x,x 2)=log p(x,x2, +) p(x,x 2, ) = σ 2 (µ+ + µ ) (µ + µ ) +x T (µ + µ ) Then, because of the conditional independence assumption, we have the correspondent naive Bayesian classifier f nb f nb (x,x 2)= σ 2 (µ+ µ )x + σ 2 (µ+ 2 µ 2 )x 2 Assume that ( ) σ σ2 = σ σ 2 X and X 2 are independent if σ 2 =0Ifσ σ 2,we have ( σ ) σ 2 σ = 2 2 σ2 σ 2 2 σ2 σ 2 σ 2 2 σ2 σ σ 2 2 σ2

5 Note that, an example E is classified into the positive class by f b, if and only if f b 0 f nb is similar Thus, when f b or f nb is divided by a non-zero positive constant, the resulting classifier is the same as f b or f nb Then, and f nb (x,x 2)=(µ + µ )x +(µ + 2 µ 2 )x 2, (9) f b (x,x 2)= = σ2 2 (σ2(µ+ σ2 2 µ 2 ) σ(µ + µ ))x + σ2 2 (σ2(µ+ σ2 µ ) σ(µ + 2 µ 2 ))x 2 +a, (0) where a = σ 2 (µ + + µ ) (µ + µ ), a constant independent of x For any x and x 2, naive Bayes has the same classification as that of the underlying classifier if That is, f b (x,x 2 )f nb (x,x 2 ) 0 () σ 2 2 σ2 ((σ2(µ+ µ )(µ + 2 µ 2 ) σ(µ + µ ) 2 )x 2 +(σ 2(µ + µ )(µ + 2 µ 2 ) σ(µ + 2 µ 2 ) 2 )x 2 2 +(2σ 2(µ + µ )(µ + 2 µ 2 ) σ((µ + µ ) 2 +(µ + 2 µ 2 ) 2 ))x x 2) +a(µ + µ )x + a(µ + 2 µ 2 )x 2 0 (2) Equation 2 represents a sufficient and necessary condition for f nb (x,x 2 ) = f b (x,x 2 ) But it is too complicated Let (µ + µ )=(µ+ 2 µ 2 ) Equation 2 is simplified as below w (x + x 2 ) 2 + w 2 (x + x 2 ) 0, (3) where w = (µ+ µ )2 σ, and w 2+σ 2 = a(µ + µ ) Let x = x + x 2, and y = w (x + x 2 ) 2 + w 2 (x + x 2 ) Figure 3 shows the area in which naive Bayes has the same classification with the target classifier Figure 3 shows that, under certain condition, naive Bayes is optimal The following theorem presents a sufficient condition for that naive Bayes works exactly as the target classifier Theorem 3 f b = fnb, if one of the following two conditions is true: µ + = µ 2, µ = µ+ 2, and σ 2 + σ>0 2 µ + = µ 2, µ+ 2 = µ, and σ 2 σ>0 Proof: () If µ + = µ 2, µ = µ+ 2, then (µ+ µ )= (µ + 2 µ 2 ) It is straightforward to verify that σ (µ µ ) (µ + µ ) = 0 That is, for the constant a in Equation 0, we have a =0 Since σ 2 + σ>0, Equation 3 is always true for any x and x 2 Therefore, f b = fnb y y=wx+w2 Figure 3: Naive Bayes has the same classification with that of the target classifier in the shaded area (2) If µ + = µ 2, µ+ 2 = µ, then (µ+ µ )= (µ+ 2 µ 2 ), and a =0 Thus, Equation 2 is simplified as below (µ + µ )2 σ 2 σ (x + x2)2 0, (4) It is obvious that Equation 4 is true for any x and x 2,if σ 2 σ>0 Therefore, f b = fnb Theorem 3 represents an explicit condition that naive Bayes is optimal It shows that naive Bayes is still optimal under certain condition, even though the conditional independence assumption is violated In other words, the conditional independence assumption is not the necessary condition for the optimality of naive Bayes This provides evidence that the dependence distribution may play the crucial role in classification Theorem 3 gives a strict condition that naive Bayes performs exactly as the target classifier In reality, however, it is not necessary to satisfy such a condition It is more practical to explore when naive Bayes works well by investigating what factors affect the performance of naive Bayes in classification Since under the assumption that two classes have a common covariance matrix + = =, and X and X 2 have the same variance σ in both classes, both f b and f nb are linear functions, we can examine the difference between them by measuring the difference between their coefficients So we have the following definition Definition 4 Given classifiers f = n w ix i + b and f 2 = n w 2ix i + c, where b and c are constants The distance between two classifiers, denoted by D(f,f 2 ),is defined as below D(f,f 2)= n (w i w 2i) 2 +(b c) 2 D(f,f 2 ) = 0, if and only if they have the same coefficients Obviously, naive Bayes will well approximate the target classifier, if the distance between them are small Therefore, we can explore when naive Bayes works well by observing what factors affect the distance x

6 When (σ 2 2 σ 2 ) > 0, f b can be simplified as below and f b (x,x 2)=(σ 2(µ + 2 µ 2 ) σ(µ + µ ))x +(σ 2(µ + µ ) σ(µ + 2 µ 2 ))x 2 + a(σ 2 2 σ 2 ), Let r = µ+ 2 µ 2 If(µ + µ + µ µ ) > 0, then f nb (x,x 2)=x + rx 2, f b (x,x 2)=(σ 2r σ)x +(σ 2 σr)x 2 + a(σ2 2 σ 2 ) µ + µ Then, D(f b,f nb )=( σ 2r+σ) 2 +(r σ 2+σr) 2 + a2 (σ2 2 σ 2 ) 2 (µ + µ )2 (5) It is easy to verify that, when (µ + µ ) < 0, we can get the same D(f b,f nb ) in Equation 5 Similarly, when (σ2 2 σ 2 ) < 0, wehave D(f b,f nb )=(+σ 2r σ) 2 +(r+σ 2 σr) 2 + a2 (σ2 2 σ 2 ) 2 (µ + µ )2 (6) From Equation 5 and 6, we see that D(f b,f nb ) is affected by r, σ 2 and σ It is true that D(f b,f nb ) increases, as r increases That means, the absolute ratio of distances between two means of classes affect significantly the performance of naive Bayes More precisely, the less absolute ratio, the better performance of naive Bayes Conclusions In this paper, we propose a new explanation on the classification performance of naive Bayes We show that, essentially, the dependence distribution; ie, how the local dependence of a node distributes in each class, evenly or unevenly, and how the local dependencies of all nodes work together, consistently (support a certain classification) or inconsistently (cancel each other out), plays a crucial role in the classification We explain why even with strong dependencies, naive Bayes still works well; ie, when those dependencies cancel each other out, there is no influence on the classification In this case, naive Bayes is still the optimal classifier In addition, we investigated the optimality of naive Bayes under the Gaussian distribution, and presented the explicit sufficient condition under which naive Bayes is optimal, even though the conditional independence assumption is violated Friedman, J 996 On bias, variance, 0/-loss, and the curse of dimensionality Data Mining and Knowledge Discovery Garg, A, and Roth, D 200 Understanding probabilistic plassifiers In Raedt, L D, and Flach, P, eds, Proceedings of 2th European Conference on Machine Learning Springer 79 9 Hand, D J, and Yu, Y 200 Idiots Bayes - not so stupid after all? International Statistical Review 69: Kononenko, I 990 Comparison of inductive and naive Bayesian learning approaches to automatic knowledge acquisition In Wielinga, B, ed, Current Trends in Knowledge Acquisition IOS Press Langley, P; Iba, W; and Thomas, K 992 An analysis of Bayesian classifiers In Proceedings of the Tenth National Conference of Artificial Intelligence AAAI Press Monti, S, and Cooper, G F 999 A Bayesian network classifier that combines a finite mixture model and a Naive Bayes model In Proceedings of the 5th Conference on Uncertainty in Artificial Intelligence Morgan Kaufmann Pazzani, M J 996 Search for dependencies in Bayesian classifiers In Fisher, D, and Lenz, H J, eds, Learning from Data: Artificial Intelligence and Statistics V Springer Verlag Quinlan, J 993 C45: Programs for Machine Learning Morgan Kaufmann: San Mateo, CA Rachlin, J R; Kasif, S; and Aha, D W 994 Toward a better understanding of memory-based reasoning systems In Proceedings of the Eleventh International Machine Learning Conference Morgan Kaufmann Roth, D 999 Learning in natural language In Proceedings of IJCAI 99 Morgan Kaufmann Zhang, H, and Ling, C X 200 Learnability of augmented Naive Bayes in nominal domains In Brodley, C E, and Danyluk, A P, eds, Proceedings of the Eighteenth International Conference on Machine Learning Morgan Kaufmann References Bennett, P N 2000 Assessing the calibration of Naive Bayes posterior estimates In Technical Report No CMU- CS00-55 Domingos, P, and Pazzani, M 997 Beyond independence: Conditions for the optimality of the simple Bayesian classifier Machine Learning 29:03 30 Frank, E; Trigg, L; Holmes, G; and Witten, I H 2000 Naive Bayes for regression Machine Learning 4():5 5

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