2018 CS420, Machine Learning, Lecture 5. Tree Models. Weinan Zhang Shanghai Jiao Tong University

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1 2018 CS420, Machine Learning, Lecture 5 Tree Models Weinan Zhang Shanghai Jiao Tong University

2 ML Task: Function Approximation Problem setting Instance feature space Instance label space Y Unknown underlying function (target) Set of function hypothesis f : 7! Y H = fhjh : 7! Yg Input: training data generated from the unknown f(x (i) ;y (i) )g = f(x (1) ;y (1) );:::;(x (n) ;y (n) )g Output: a hypothesis h 2 H that best approximates Optimize in functional space, not just parameter space f

3 Optimize in Functional Space Tree models Intermediate node for splitting data Leaf node for label prediction Continuous data example x 2 Class 1 Class 2 x 1 < a 1 Root Node a 3 a 2 Yes No x 2 < a 2 x 2 < a 3 Intermediate Node Class 2 Class 1 Yes No Yes No a 1 x 1 y = -1 y = 1 y = 1 y = -1 Leaf Node

4 Optimize in Functional Space Tree models Intermediate node for splitting data Leaf node for label prediction Discrete/categorical data example Outlook Root Node Sunny Overcast Rain Humidity y = 1 Wind Intermediate Node Leaf High Normal Strong Weak Node y = -1 y = 1 y = -1 y = 1 Leaf Node

5 Decision Tree Learning Problem setting Instance feature space Instance label space Y Unknown underlying function (target) Set of function hypothesis f : 7! Y H = fhjh : 7! Yg Input: training data generated from the unknown f(x (i) ;y (i) )g = f(x (1) ;y (1) );:::;(x (n) ;y (n) )g Output: a hypothesis h 2 H that best approximates Here each hypothesis is a decision tree h f

6 Decision Tree Decision Boundary Decision trees divide the feature space into axisparallel (hyper-)rectangles Each rectangular region is labeled with one label or a probabilistic distribution over labels Slide credit: Eric Eaton

7 History of Decision-Tree Research Hunt and colleagues used exhaustive search decision-tree methods (CLS) to model human concept learning in the 1960 s. In the late 70 s, Quinlan developed ID3 with the information gain heuristic to learn expert systems from examples. Simultaneously, Breiman and Friedman and colleagues developed CART (Classification and Regression Trees), similar to ID3. In the 1980 s a variety of improvements were introduced to handle noise, continuous features, missing features, and improved splitting criteria. Various expert-system development tools results. Quinlan s updated decision-tree package (C4.5) released in Sklearn (python)weka (Java) now include ID3 and C4.5 Slide credit: Raymond J. Mooney

8 Decision Trees Tree models Intermediate node for splitting data Leaf node for label prediction Key questions for decision trees How to select node splitting conditions? How to make prediction? How to decide the tree structure?

9 Node Splitting Which node splitting condition to choose? Outlook Temperature Sunny Overcast Rain Hot Mild Cool Choose the features with higher classification capacity Quantitatively, with higher information gain

10 Fundamentals of Information Theory Entropy (more specifically, Shannon entropy) is the expected value (average) of the information contained in each message. Suppose is a random variable with n discrete values then its entropy H() is P ( = x i )=p i H() = n p i log p i i=1 It is easy to verify n H() = p i log p i i=1 n i=1 1 n log 1 n =logn

11 Illustration of Entropy Entropy of binary distribution H() = p 1 log p 1 (1 p 1 )log(1 p 1 )

12 Cross Entropy Cross entropy is used to measure the difference between two random variable distributions H(; Y )= n P ( = i)logp (Y = i) i=1 Continuous formulation Z H(p; q) = Compared to KL divergence p(x)logq(x)dx Z D KL(pkq) = p(x)log p(x) dx = H(p; q) H(p) q(x)

13 KL-Divergence Kullback Leibler divergence (also called relative entropy) is a measure of how one probability distribution diverges from a second, expected probability distribution Z D KL(pkq) = p(x)log p(x) dx = H(p; q) H(p) q(x)

14 Review Cross Entropy in Logistic Regression Logistic regression is a binary classification model p μ (y =1jx) =¾(μ > x)= p μ (y =0jx) = e μ> x 1+e μ> x 1 1+e μ> x Cross entropy loss function Gradient L(y;x; p μ )= ylog ¾(μ > x) (1 y)log(1 ¾(μ > x)) x; p μ ) 1 = ¾(μ > x) ¾(z)(1 ¾(z))x (1 y) 1 1 ¾(μ > ¾(z)(1 ¾(z))x x) =(¾(μ > x) μ Ã μ +(y ¾(μ > x))x = ¾(z)(1 x

15 Conditional Entropy Entropy H() = n P ( = i) log P ( = i) i=1 Specific conditional entropy of given Y = v H(jY = v) = n P ( = ijy = v)logp ( = ijy = v) i=1 Specific conditional entropy of given Y H(jY )= v2values(y ) P (Y = v)h(jy = v) Information Gain or Mutual Information of given Y I(; Y )=H() H(jY )=H(Y ) H(Y j) =H()+H(Y ) H(; Y ) Entropy of (,Y) instead of cross entropy

16 Information Gain Information Gain or Mutual Information of given Y I(; Y )=H() H(jY ) = P ( = v)logp ( = v)+ v u = P ( = v)logp ( = v)+ v u = P ( = v)logp ( = v)+ v u = P ( = v)logp ( = v) v v P (Y = u) P ( = vjy = u)logp ( = vjy = u) v P ( = v; Y = u)logp ( = vjy = u) v P ( = v; Y = u)[log P ( = v; Y = u) log P (Y = u)] v P (Y = v)logp (Y = v)+ P ( = v; Y = u)logp ( = v; Y = u) u;v =H()+H(Y ) H(; Y ) Entropy of (,Y) instead of cross entropy

17 Node Splitting Information gain H(jY = v) = H(jY )= n P ( = ijy = v)logp ( = ijy = v) i=1 v2values(y ) P (Y = v)h(jy = v) Outlook Temperature Sunny Overcast Rain Hot Mild Cool H(jY = S) = 3 5 log log 2 5 =0:9710 H(jY = O) = 4 4 log 4 4 =0 H(jY = R) = 4 5 log log 1 5 =0:7219 H(jY = H) = 2 4 log log 2 4 =1 H(jY = M) = 1 4 log log 3 4 =0:8113 H(jY = C) = 4 6 log log 2 6 =0:9183 H(jY )= : :7219 = 0:6046 H(jY )= : :9183 = 0:9111 I(; Y )=H() H(jY )=1 0:6046 = 0:3954 I(; Y )=H() H(jY )=1 0:9111 = 0:0889

18 Information Gain Ratio The ratio between information gain and the entropy I R (; Y )= I(; Y ) H Y () H() H(jY ) = H Y () where the entropy (of Y) is H Y () = v2values(y ) j y=v j log j y=vj jj jj where j y=v j is the number of observations with the feature y=v

19 Node Splitting Information gain ratio I R (; Y )= I(; Y ) H Y () H() H(jY ) = H Y () Outlook Temperature Sunny Overcast Rain Hot Mild Cool I(; Y )=H() H(jY )=1 0:6046 = 0:3954 H Y () = 5 14 log log log 5 14 =1:5774 I R (; Y )= 0:3954 1:5774 =0:2507 I(; Y )=H() H(jY )=1 0:9111 = 0:0889 H Y () = 4 14 log log log 6 14 =1:5567 I R (; Y )= 0:0889 1:5567 =0:0571

20 Decision Tree Building: ID3 Algorithm Algorithm framework Start from the root node with all data For each node, calculate the information gain of all possible features Choose the feature with the highest information gain Split the data of the node according to the feature Do the above recursively for each leaf node, until There is no information gain for the leaf node Or there is no feature to select

21 Decision Tree Building: ID3 Algorithm An example decision tree from ID3 Outlook Sunny Overcast Rain Temperature Hot Mild Cool Each path only involves a feature at most once

22 Decision Tree Building: ID3 Algorithm An example decision tree from ID3 Outlook Sunny Overcast Rain Temperature Wind Hot Mild Cool Strong Weak How about this tree, yielding perfect partition?

23 Overfitting Tree model can approximate any finite data by just growing a leaf node for each instance Outlook Sunny Overcast Rain Temperature Wind Hot Mild Cool Strong Weak

24 Decision Tree Training Objective Cost function of a tree T over training data jt j C(T )= N t H t (T ) t=1 where for the leaf node t H t (T) is the empirical entropy N t is the instance number, N tk is the instance number of class k H t (T )= k N tk log N tk N t N t Training objective: find a tree to minimize the cost T jt j min C(T )= N t H t (T ) t=1

25 Decision Tree Regularization Cost function over training data C(T )= jt j N t H t (T )+ jt j t=1 where T is the number of leaf nodes of the tree T λ is the hyperparameter of regularization

26 Decision Tree Building: ID3 Algorithm An example decision tree from ID3 Outlook Sunny Overcast Rain Hot Temperature Mild Cool Strong Wind Weak Whether to split this node? Calculate the cost function difference. C(T )= jt j N t H t (T )+ jt j t=1

27 Summary of ID3 A classic and straightforward algorithm of training decision trees Work on discrete/categorical data One branch for each value/category of the feature Algorithm C4.5 is similar and more advanced to ID3 Splitting the node according to information gain ratio Splitting branch number depends on the number of different categorical values of the feature Might lead to very broad tree

28 CART Algorithm Classification and Regression Tree (CART) Proposed by Leo Breiman et al. in 1984 Binary splitting (yes or no for the splitting condition) Can work on continuous/numeric features Can repeatedly use the same feature (with different splitting) Condition 1 Yes No Condition 2 Prediction 3 Yes No Prediction 1 Prediction 2

29 CART Algorithm Regression Tree Output the predicted value Classification Tree Output the predicted class Age > 20 Age > 20 Yes No Yes No Gender=Male 2.8 Gender=Male dislike Yes No Yes No For example: predict the user s rating to a movie like dislike For example: predict whether the user like a move

30 Regression Tree Let the training dataset with continuous targets y D = f(x 1 ;y 1 ); (x 2 ;y 2 );:::;(x N ;y N )g Suppose a regression tree has divided the space into M regions R 1, R 2,, R M, with c m as the prediction for region R m M f(x) = c m I(x 2 R m ) m=1 Loss function for (x i, y i ) 1 2 (y i f(x i )) 2 It is easy to see the optimal prediction for region m is ^c m =avg(y i jx i 2 R m )

31 Regression Tree How to find the optimal splitting regions? How to find the optimal splitting conditions? Defined by a threshold value s on variable j Lead to two regions R 1 (j; s) =fxjx (j) sg R 2 (j; s) =fxjx (j) >sg Training based on current splitting h min min j;s c 1 x2r 1 (j;s) (y i c 1 ) 2 +min c 2 x2r 2 (j;s) (y i c 2 ) 2i ^c m =avg(y i jx i 2 R m )

32 Regression Tree Algorithm INPUT: training data D OUTPUT: regression tree f(x) Repeat until stop condition satisfied: Find the optimal splitting (j,s) h min min j;s c 1 x2r 1 (j;s) (y i c 1 ) 2 +min c 2 x2r 2 (j;s) (y i c 2 ) 2i Calculate the prediction value of the new region R 1, R 2 ^c m =avg(y i jx i 2 R m ) Return the regression tree f(x) = M m=1 ^c m I(x 2 R m )

33 Regression Tree Algorithm How to efficiently find the optimal splitting (j,s)? h min min j;s c 1 x2r 1 (j;s) (y i c 1 ) 2 +min c 2 x2r 2 (j;s) (y i c 2 ) 2i Sort the data ascendingly according to feature j value Splitting threshold s y 1 y 2 y 3 y 4 y 5 y 6 y 7 y 8 y 9 y 10 y 11 y 12 small j value large j value 6 12 loss = (y i c 1 ) 2 + (y i c 2 ) 2 = i=1 i=7 6 y i 2 1 ³ 6 12 y i 2 + yi i=1 i=1 i=7 ³ 12 1 ³ 6 1 y i 2 = y i i=7 Online updated i=1 ³ 12 y i 2 + C i=7

34 Regression Tree Algorithm How to efficiently find the optimal splitting (j,s)? h min min j;s c 1 x2r 1 (j;s) (y i c 1 ) 2 +min c 2 x2r 2 (j;s) (y i c 2 ) 2i Sort the data ascendingly according to feature j value Splitting threshold s y 1 y 2 y 3 y 4 y 5 y 6 y 7 y 8 y 9 y 10 y 11 y 12 small j value large j value ³ 1 loss6;7 = 6 6 ³ 1 12 y i 2 y i 2 + C 6 i=1 i=7

35 Regression Tree Algorithm How to efficiently find the optimal splitting (j,s)? h min min j;s c 1 x2r 1 (j;s) (y i c 1 ) 2 +min c 2 x2r 2 (j;s) (y i c 2 ) 2i Sort the data ascendingly according to feature j value Splitting threshold s y 1 y 2 y 3 y 4 y 5 y 6 y 7 y 8 y 9 y 10 y 11 y 12 small j value large j value loss6;7 = 1 ³ 6 1 y i i=1 loss7;8 = 1 ³ 7 1 y i i=1 ³ 12 y i 2 + C i=7 ³ 12 y i 2 + C Maintain and online update in O(1) Time y i 2 + C i=8 k n Sum(R 1 )= y i Sum(R 2 )= i=1 i=k+1 O(n) in total for checking one feature y i

36 Classification Tree The training dataset with categorical targets y D = f(x 1 ;y 1 ); (x 2 ;y 2 );:::;(x N ;y N )g Suppose a regression tree has divided the space into M regions R 1, R 2,, R M, with c m as the prediction for region R m M f(x) = c m I(x 2 R m ) m=1 Here the leaf node prediction c m is the category distribution ^c m = fp (y k jx i 2 R m )g k=1:::k c m is solved by counting categories P (y k jx i 2 R m )= Ck m C m # instances in leaf m with cat k # instances in leaf m

37 Classification Tree How to find the optimal splitting regions? How to find the optimal splitting conditions? For continuous feature j, defined by a threshold value s Yield two regions R 1 (j; s) =fxjx (j) sg R 2 (j; s) =fxjx (j) >sg For categorical feature j, select a category a Yield two regions R 1 (j; s) =fxjx (j) = ag R 2 (j; s) =fxjx (j) 6= ag How to select? Argmin Gini impurity.

38 Gini Impurity In classification problem suppose there are K classes let p k be the probability of an instance with the class k the Gini impurity index is Gini(p) = K p k (1 p k )=1 k=1 K k=1 Given the training dataset D, the Gini impurity is p 2 k Gini(D) =1 K ³ jd k j jdj k=1 2 # instances in D with cat k # instances in D

39 Gini Impurity For binary classification problem let p be the probability of an instance with the class 1 Gini impurity is Gini(p) = 2p(1 p) Entropy is H(p) = p log p (1 p)log(1 p) Gini impurity and entropy are quite similar in representing classification error rate.

40 Gini Impurity With a categorical feature j and one of its categories a The two split regions R 1, R 2 R 1 (j; a) =fxjx (j) = ag R 2 (j; a) =fxjx (j) 6= ag D j 1 = f(x; y)jx (j) = ag D j 2 = f(x; y)jx (j) 6= ag The Gini impurity of feature j with the selected category a Gini(D j ;j = a) = jd1 j j jd j j Gini(D1 j )+ jd2 j j jd j j Gini(D2 j )

41 Classification Tree Algorithm INPUT: training data D OUTPUT: classification tree f(x) Repeat until stop condition satisfied: Find the optimal splitting (j,a) min Gini(D j;j = a) j;a 1. Node instance number is small 2. Gini impurity is small 3. No more feature Calculate the prediction distribution of the new region R 1, R 2 ^c m = fp (y k jx i 2 R m )g k=1:::k Return the classification tree f(x) = M m=1 ^c m I(x 2 R m )

42 Classification Tree Output Class label output Output the class with the highest conditional probability f(x) =argmax y k m=1 Probabilistic distribution output M I(x 2 R m )P (y k jx i 2 R m ) f(x) = M m=1 ^c m I(x 2 R m ) ^c m = fp (y k jx i 2 R m )g k=1:::k

43 Converting a Tree to Rules Yes Yes Gender=Male Age > 20 No No 2.8 For example: predict the user s rating to a movie IF Age > 20: IF Gender == Male: return 4.8 ELSE: return 4.1 ELSE: return 2.8 Decision tree model is easy to be visualized, explained and debugged.

44 Learning Model Comparison [Table 10.3 from Hastie et al. Elements of Statistical Learning, 2nd Edition]

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