1. CHAPTER 9: Decision Trees
Slides corrected and new slides added by Ch. Eick

2. Tree Uses Nodes, and Leaves
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

3. Divide and Conquer Internal decision nodes
Univariate: Uses a single attribute, xi
Numeric xi : Binary split : xi > wm
Discrete xi : n-way split for n possible values
Multivariate: Uses all attributes, x
Leaves
Classification: Class labels, or proportions
Regression: Numeric; r average, or local fit
Learning is greedy; find the best split recursively (Breiman et al, 1984; Quinlan, 1986, 1993) without backtracking (decision taken are final)

4. Side Discussion “Greedy Algorithms”
Fast and therefore attractive to solve NP-hard and other problems with high complexity. Later decisions are made in the context of decision selected early dramatically reducing the size of the search space.
They do not backtrack: if they make a bad decision (based on local criteria), they never revise the decision.
They are not guaranteed to find the optimal solutions, and sometimes can get deceived and find really bad solutions.
In spite of what is said above, a lot successful and popular algorithms in Computer Science are greedy algorithms.
Greedy algorithms are particularly popular in AI and Operations Research.
Popular Greedy Algorithms: Decision Tree Induction,…

5. Classification Trees (ID3, CART, C4.5)
For node m, Nm instances reach m, Nim belong to Ci
Node m is pure if pim is 0 or 1
Measure of impurity is entropy
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

6. Best Split
skip
If node m is pure, generate a leaf and stop, otherwise split and continue recursively
Impurity after split: Nmj of Nm take branch j. Nimj belong to Ci
Find the variable and split that min impurity (among all variables -- and split positions for numeric variables)
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

7. Tree Induction Greedy strategy. Issues
Split the records based on an attribute test that optimizes certain criterion.
Issues
Determine how to split the records
How to specify the attribute test condition?
How to determine the best split?
Determine when to stop splitting

8. Information Gain vs. Gain Ratio
Result:
I_Gain_Ratio:
city>age>car
Result:
I_Gain:
age > car=city
Gain(D,city=)= H(1/3,2/3) – ½ H(1,0) –
½ H(1/3,2/3)=0.45
D=(2/3,1/3)
G_Ratio_pen(city=)=H(1/2,1/2)=1
city=sf
city=la
D1(1,0)
D2(1/3,2/3)
Gain(D,car=)= H(1/3,2/3) – 1/6 H(0,1) –
½ H(2/3,1/3) – 1/3 H(1,0)=0.45
G_Ratio_pen(car=)=H(1/2,1/3,1/6)=1.45
D=(2/3,1/3)
car=van
car=merc
car=taurus
D1(0,1)
D2(2/3,1/3)
D3(1,0)
G_Ratio_pen(age=)=log2(6)=2.58
Gain(D,age=)= H(1/3,2/3) – 6*1/6 H(0,1)
= 0.90
D=(2/3,1/3)
age=22
age=25
age=27
age=35
age=40
age=50
D1(1,0)
D2(0,1)
D3(1,0)
D4(1,0)
D5(1,0)
D6(0,1)

9. How to determine the Best Split?
Before Splitting: 10 records of class 0, 10 records of class 1
Before: E(1/2,1/2)
After: 4/20*E(1/4,3/4) + 8/20*E(1,0) + 8/20*E(1/8,7/8)
Gain: Before-After
Pick Test that has the highest gain!
Remark: E stands for Gini, Entropy(H), Impurity(1-maxc(P(c )), Gain-ratio

10. Entropy and Gain Computations
Assume we have m classes in our classification problem. A test S subdivides the examples D= (p1,…,pm) into n subsets D1 =(p11,…,p1m) ,…,Dn =(p11,…,p1m). The qualify of S is evaluated using Gain(D,S) (ID3) or GainRatio(D,S) (C5.0):
Let H(D=(p1,…,pm))= Si=1 (pi log2(1/pi)) (called the entropy function)
Gain(D,S)= H(D) - Si=1 (|Di|/|D|)*H(Di)
Gain_Ratio(D,S)= Gain(D,S) / H(|D1|/|D|,…, |Dn|/|D|)
Remarks:
|D| denotes the number of elements in set D.
D=(p1,…,pm) implies that p1+…+ pm =1 and indicates that of the |D| examples p1*|D| examples belong to the first class, p2*|D| examples belong to the second class,…, and pm*|D| belong the m-th (last) class.
H(0,1)=H(1,0)=0; H(1/2,1/2)=1, H(1/4,1/4,1/4,1/4)=2, H(1/p,…,1/p)=log2(p).
C5.0 selects the test S with the highest value for Gain_Ratio(D,S), whereas ID3 picks the test S for the examples in set D with the highest value for Gain (D,S). m n

11. Information Gain vs. Gain Ratio
Result:
I_Gain_Ratio:
city>age>car
Result:
I_Gain:
age > car=city
Gain(D,city=)= H(1/3,2/3) – ½ H(1,0) –
½ H(1/3,2/3)=0.45
D=(2/3,1/3)
G_Ratio_pen(city=)=H(1/2,1/2)=1
city=sf
city=la
D1(1,0)
D2(1/3,2/3)
Gain(D,car=)= H(1/3,2/3) – 1/6 H(0,1) –
½ H(2/3,1/3) – 1/3 H(1,0)=0.45
G_Ratio_pen(car=)=H(1/2,1/3,1/6)=1.45
D=(2/3,1/3)
car=van
car=merc
car=taurus
D1(0,1)
D2(2/3,1/3)
D3(1,0)
G_Ratio_pen(age=)=log2(6)=2.58
Gain(D,age=)= H(1/3,2/3) – 6*1/6 H(0,1)
= 0.90
D=(2/3,1/3)
age=22
age=25
age=27
age=35
age=40
age=50
D1(1,0)
D2(0,1)
D3(1,0)
D4(1,0)
D5(1,0)
D6(0,1)

12. Splitting Continuous Attributes
Different ways of handling
Discretization to form an ordinal categorical attribute
Static – discretize once at the beginning
Dynamic – ranges can be found by equal interval bucketing, equal frequency bucketing (percentiles), clustering, or supervised
clustering.
Binary Decision: (A < v) or (A  v)
consider all possible splits and finds the best cut v
can be more compute intensive

13. Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

14. Stopping Criteria for Tree Induction
Grow entire tree
Stop expanding a node when all the records belong to the same class
Stop expanding a node when all the records have the same attribute values
Pre-pruning (do not grow complete tree)
Stop when only x examples are left (pre-pruning)
… other pre-pruning strategies

15. How to Address Overfitting in Decision Trees
The most popular approach: Post-pruning
Grow decision tree to its entirety
Trim the nodes of the decision tree in a bottom-up fashion
If generalization error improves after trimming, replace sub-tree by a leaf node.
Class label of leaf node is determined from majority class of instances in the sub-tree

16. Advantages Decision Tree Based Classification
Inexpensive to construct
Extremely fast at classifying unknown records
Easy to interpret for small-sized trees
Okay for noisy data
Can handle both continuous and symbolic attributes
Accuracy is comparable to other classification techniques for many simple data sets
Decent average performance over many datasets
Kind of a standard—if you want to show that your “new” classification technique really “improves the world”  compare its performance against decision trees (e.g. C 5.0) using 10-fold cross-validation
Does not need distance functions; only the order of attribute values is important for classification: 0.1,0.2,0.3 and 0.331,0.332, and is the same for a decision tree learner.

17. Disadvantages Decision Tree Based Classification
Relies on rectangular approximation that might not be good for some dataset
Selecting good learning algorihtm parameters (e.g. degree of pruning) is non-trivial
Ensemble techniques, support vector machines, and kNN might obtain higher accuracies for a specific dataset.
More recently, forests (ensembles of decision trees) have gained some popularity.

18. Error in Regressions Trees
Examples are:
(1,1.37), (2,-1.45), ( ), (2.2,-125)
(7, 2.06), (8, 1.66)
Error before split: 1/6(2*0**2+ 2* *0.04)
In general: minimizes squared error!

19. Regression Trees Error at node m: After splitting:
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

20. Error in Regressions Trees
Examples are:
(1,1.37), (2,-1.45), ( ), (2.2,-125)
(7, 2.06), (8, 1.66)
Error before split: 1/6(2*0**2+ 2* *0.04)

21. Model Selection in Trees:
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)

22. Summary Regression Trees
Regression trees work as decision trees except
Leafs carry numbers which predict the output variable instead of class label
Instead of entropy/purity the squared prediction error serves as the performance measure.
Tests that reduce the squared prediction error the most are selected as node test; test conditions have the form y>threshold where y is an independent variable of the prediction problem.
Instead of using the majority class, leaf labels are generated by averaging over the values of the dependent variable of the examples that are associated with the particular class node
Like decision trees, regression tree employ rectangular tessellations with a fixed number being associated with each rectangle; the number is the output for the inputs which lie inside the rectangle.
In contrast to ordinary regression that performs curve fitting, regression tries use averaging with respect to the output variable when making predictions.

23. Multivariate Trees
Lecture Notes for E Alpaydın 2004 Introduction to Machine Learning © The MIT Press (V1.1)