1. Decision Tree Classifier10 1 2 3 4 5 6 7 8 9
Ross Quinlan
Abdomen Length > 7.1?
Antenna Length no
yes
Antenna Length > 6.0?
Katydid no
yes
Grasshopper
Katydid
Abdomen Length

2. Antennae shorter than body?
Yes No
3 Tarsi?
Grasshopper
Yes No
Foretiba has ears?
Yes No
Cricket
Decision trees predate computers
Katydids
Camel Cricket

3. Decision Tree Classification
A flow-chart-like tree structure
Internal node denotes a test on an attribute
Branch represents an outcome of the test
Leaf nodes represent class labels or class distribution
Decision tree generation consists of two phases
Tree construction
At start, all the training examples are at the root
Partition examples recursively based on selected attributes
Tree pruning
Identify and remove branches that reflect noise or outliers
Use of decision tree: Classifying an unknown sample
Test the attribute values of the sample against the decision tree

4. How do we construct the decision tree?
Basic algorithm (a greedy algorithm)
Tree is constructed in a top-down recursive divide-and-conquer manner
At start, all the training examples are at the root
Attributes are categorical (if continuous-valued, they can be discretized in advance)
Examples are partitioned recursively based on selected attributes.
Test attributes are selected on the basis of a heuristic or statistical measure (e.g., information gain)
Conditions for stopping partitioning
All samples for a given node belong to the same class
There are no remaining attributes for further partitioning – majority voting is employed for classifying the leaf
There are no samples left

5. Information Gain as A Splitting Criteria
Select the attribute with the highest information gain (information gain is the expected reduction in entropy).
Assume there are two classes, P and N
Let the set of examples S contain p elements of class P and n elements of class N
The amount of information, needed to decide if an arbitrary example in S belongs to P or N is defined as
0 log(0) is defined as 0

6. Information Gain in Decision Tree Induction
Assume that using attribute A, a current set will be partitioned into some number of child sets
The encoding information that would be gained by branching on A
Note: entropy is at its minimum if the collection of objects is completely uniform

7. Person Homer 0” 250 36 M Marge 10” 150 34 F Bart 2” 90 10 Lisa 6” 78 8
Hair Length
Weight
Age
Class
Homer 0”
250 36 M
Marge
10”
150 34 F
Bart 2” 90 10
Lisa 6” 78 8
Maggie 4” 20 1
Abe 1”
170 70
Selma 8”
160 41
Otto
180 38
Krusty
200 45
Comic 8”
290 38 ?

8. Let us try splitting on Hair length
Entropy(4F,5M) = -(4/9)log2(4/9) - (5/9)log2(5/9) =
yes no
Hair Length <= 5?
Let us try splitting on Hair length
Entropy(1F,3M) = -(1/4)log2(1/4) - (3/4)log2(3/4) =
Entropy(3F,2M) = -(3/5)log2(3/5) - (2/5)log2(2/5) =
Gain(Hair Length <= 5) = – (4/9 * /9 * ) =

9. Let us try splitting on Weight
Entropy(4F,5M) = -(4/9)log2(4/9) - (5/9)log2(5/9) =
yes no
Weight <= 160?
Let us try splitting on Weight
Entropy(4F,1M) = -(4/5)log2(4/5) - (1/5)log2(1/5) =
Entropy(0F,4M) = -(0/4)log2(0/4) - (4/4)log2(4/4)
= 0
Gain(Weight <= 160) = – (5/9 * /9 * 0 ) =

10. Let us try splitting on Age
Entropy(4F,5M) = -(4/9)log2(4/9) - (5/9)log2(5/9) =
yes no
age <= 40?
Let us try splitting on Age
Entropy(3F,3M) = -(3/6)log2(3/6) - (3/6)log2(3/6)
= 1
Entropy(1F,2M) = -(1/3)log2(1/3) - (2/3)log2(2/3) =
Gain(Age <= 40) = – (6/9 * 1 + 3/9 * ) =

11. This time we find that we can split on Hair length, and we are done!
Of the 3 features we had, Weight was best. But while people who weigh over 160 are perfectly classified (as males), the under 160 people are not perfectly classified… So we simply recurse!
yes no
Weight <= 160?
This time we find that we can split on Hair length, and we are done!
yes no
Hair Length <= 2?

12. We need don’t need to keep the data around, just the test conditions.
Weight <= 160?
yes no
How would these people be classified?
Hair Length <= 2?
Male
yes no
Male
Female

13. Male Male Female It is trivial to convert Decision Trees to rules… yes
Weight <= 160?
yes no
Hair Length <= 2?
Male
yes no
Male
Female
Rules to Classify Males/Females
If Weight greater than 160, classify as Male
Elseif Hair Length less than or equal to 2, classify as Male
Else classify as Female

14. Once we have learned the decision tree, we don’t even need a computer!
This decision tree is attached to a medical machine, and is designed to help nurses make decisions about what type of doctor to call.
Decision tree for a typical shared-care setting applying the system for the diagnosis of prostatic obstructions.

15. The worked examples we have seen were performed on small datasets
The worked examples we have seen were performed on small datasets. However with small datasets there is a great danger of overfitting the data…
When you have few datapoints, there are many possible splitting rules that perfectly classify the data, but will not generalize to future datasets.
Yes No
Wears green?
Female
Male
For example, the rule “Wears green?” perfectly classifies the data, so does “Mothers name is Jacqueline?”, so does “Has blue shoes”…

16. Avoid Overfitting in Classification
The generated tree may overfit the training data
Too many branches, some may reflect anomalies due to noise or outliers
Result is in poor accuracy for unseen samples
Two approaches to avoid overfitting
Prepruning: Halt tree construction early—do not split a node if this would result in the goodness measure falling below a threshold
Difficult to choose an appropriate threshold
Postpruning: Remove branches from a “fully grown” tree—get a sequence of progressively pruned trees
Use a set of data different from the training data to decide which is the “best pruned tree”

17. ? Which of the “Pigeon Problems” can be solved by a Decision Tree?10 1 2 3 4 5 6 7 8 9
Deep Bushy Tree
Useless ? 10 1 2 3 4 5 6 7 8 9
100 10 20 30 40 50 60 70 80 90
The Decision Tree has a hard time with correlated attributes

18. Advantages/Disadvantages of Decision Trees
Easy to understand (Doctors love them!)
Easy to generate rules
Disadvantages:
May suffer from overfitting.
Classifies by rectangular partitioning (so does not handle correlated features very well).
Can be quite large – pruning is necessary.
Does not handle streaming data easily

19. Naïve Bayes Classifier
Thomas Bayes
We will start off with a visual intuition, before looking at the math…

20. Grasshoppers Katydids Remember this example? Let’s get lots more data…10 1 2 3 4 5 6 7 8 9
Antenna Length
Abdomen Length
Remember this example? Let’s get lots more data…

21. With a lot of data, we can build a histogram
With a lot of data, we can build a histogram. Let us just build one for “Antenna Length” for now… 10 1 2 3 4 5 6 7 8 9
Antenna Length
Katydids
Grasshoppers

22. We can leave the histograms as they are, or we can summarize them with two normal distributions.
Let us us two normal distributions for ease of visualization in the following slides…

23. p(cj | d) = probability of class cj, given that we have observed d
We want to classify an insect we have found. Its antennae are 3 units long. How can we classify it?
We can just ask ourselves, give the distributions of antennae lengths we have seen, is it more probable that our insect is a Grasshopper or a Katydid.
There is a formal way to discuss the most probable classification…
p(cj | d) = probability of class cj, given that we have observed d 3
Antennae length is 3

24. p(cj | d) = probability of class cj, given that we have observed d
P(Grasshopper | 3 ) = 10 / (10 + 2) = 0.833
P(Katydid | 3 ) = 2 / (10 + 2) = 0.166 10 2 3
Antennae length is 3

25. p(cj | d) = probability of class cj, given that we have observed d
P(Grasshopper | 7 ) = 3 / (3 + 9) = 0.250
P(Katydid | 7 ) = 9 / (3 + 9) = 0.750 9 3 7
Antennae length is 7

26. p(cj | d) = probability of class cj, given that we have observed d
P(Grasshopper | 5 ) = 6 / (6 + 6) = 0.500
P(Katydid | 5 ) = 6 / (6 + 6) = 0.500 6 6 5
Antennae length is 5

27. Bayes Classifiers
That was a visual intuition for a simple case of the Bayes classifier, also called:
Idiot Bayes
Naïve Bayes
Simple Bayes
We are about to see some of the mathematical formalisms, and more examples, but keep in mind the basic idea.
Find out the probability of the previously unseen instance belonging to each class, then simply pick the most probable class.

28. Bayes Classifiers Bayesian classifiers use Bayes theorem, which says
p(cj | d ) = p(d | cj ) p(cj)
p(d)
p(cj | d) = probability of instance d being in class cj,
This is what we are trying to compute
p(d | cj) = probability of generating instance d given class cj,
We can imagine that being in class cj, causes you to have feature d with some probability
p(cj) = probability of occurrence of class cj,
This is just how frequent the class cj, is in our database
p(d) = probability of instance d occurring
This can actually be ignored, since it is the same for all classes

29. c1 = male, and c2 = female. Assume that we have two classes
We have a person whose sex we do not know, say “drew” or d.
Classifying drew as male or female is equivalent to asking is it more probable that drew is male or female, I.e which is greater p(male | drew) or p(female | drew)
(Note: “Drew can be a male or female name”)
Drew Barrymore
Drew Carey
What is the probability of being called “drew” given that you are a male?
What is the probability of being a male?
p(male | drew) = p(drew | male ) p(male)
p(drew)
What is the probability of being named “drew”? (actually irrelevant, since it is that same for all classes)

30. p(cj | d) = p(d | cj ) p(cj) p(d)
This is Officer Drew (who arrested me in 1997). Is Officer Drew a Male or Female?
Luckily, we have a small database with names and sex.
We can use it to apply Bayes rule…
Name
Sex
Drew
Male
Claudia
Female
Alberto
Karin
Nina
Sergio
Officer Drew
p(cj | d) = p(d | cj ) p(cj) p(d)

31. p(cj | d) = p(d | cj ) p(cj) p(d)
Name
Sex
Drew
Male
Claudia
Female
Alberto
Karin
Nina
Sergio
p(cj | d) = p(d | cj ) p(cj) p(d)
Officer Drew
p(male | drew) = 1/3 * 3/8 = 0.125
3/ /8
Officer Drew is more likely to be a Female.
p(female | drew) = 2/5 * 5/8 = 0.250
3/ /8

32. Officer Drew IS a female!
p(male | drew) = 1/3 * 3/8 = 0.125
3/ /8
p(female | drew) = 2/5 * 5/8 = 0.250
3/ /8

33. p(cj | d) = p(d | cj ) p(cj) p(d)
So far we have only considered Bayes Classification when we have one attribute (the “antennae length”, or the “name”). But we may have many features.
How do we use all the features?
p(cj | d) = p(d | cj ) p(cj) p(d)
Name
Over 170CM
Eye
Hair length
Sex
Drew No
Blue
Short
Male
Claudia
Yes
Brown
Long
Female
Alberto
Karin
Nina
Sergio

34. p(d|cj) = p(d1|cj) * p(d2|cj) * ….* p(dn|cj)
To simplify the task, naïve Bayesian classifiers assume attributes have independent distributions, and thereby estimate
p(d|cj) = p(d1|cj) * p(d2|cj) * ….* p(dn|cj)
The probability of class cj generating instance d, equals….
The probability of class cj generating the observed value for feature 1, multiplied by..
The probability of class cj generating the observed value for feature 2, multiplied by..

35. p(d|cj) = p(d1|cj) * p(d2|cj) * ….* p(dn|cj)
To simplify the task, naïve Bayesian classifiers assume attributes have independent distributions, and thereby estimate
p(d|cj) = p(d1|cj) * p(d2|cj) * ….* p(dn|cj)
p(officer drew|cj) = p(over_170cm = yes|cj) * p(eye =blue|cj) * ….
Officer Drew is blue-eyed, over 170cm tall, and has long hair
p(officer drew| Female) = 2/5 * 3/5 * ….
p(officer drew| Male) = 2/3 * 2/3 * ….

36. cj … p(d1|cj) p(d2|cj) p(dn|cj)
The Naive Bayes classifiers is often represented as this type of graph…
Note the direction of the arrows, which state that each class causes certain features, with a certain probability …
p(d1|cj) p(d2|cj) p(dn|cj)

37. cj … Naïve Bayes is fast and space efficient
We can look up all the probabilities with a single scan of the database and store them in a (small) table… …
p(d1|cj) p(d2|cj) p(dn|cj)
Sex
Over190cm
Male
Yes
0.15 No
0.85
Female
0.01
0.99
Sex
Long Hair
Male
Yes
0.05 No
0.95
Female
0.70
0.30
Sex
Male
Female

38. Naïve Bayes is NOT sensitive to irrelevant features...
Suppose we are trying to classify a persons sex based on several features, including eye color. (Of course, eye color is completely irrelevant to a persons gender)
p(Jessica |cj) = p(eye = brown|cj) * p( wears_dress = yes|cj) * ….
p(Jessica | Female) = 9,000/10, * 9,975/10,000 * ….
p(Jessica | Male) = 9,001/10, * 2/10, * ….
Almost the same!
However, this assumes that we have good enough estimates of the probabilities, so the more data the better.

39. cj
An obvious point. I have used a simple two class problem, and two possible values for each example, for my previous examples. However we can have an arbitrary number of classes, or feature values …
p(d1|cj) p(d2|cj) p(dn|cj)
Animal
Mass >10kg
Cat
Yes
0.15 No
0.85
Dog
0.91
0.09
Pig
0.99
0.01
Animal
Color
Cat
Black
0.33
White
0.23
Brown
0.44
Dog
0.97
0.03
0.90
Pig
0.04
0.01
0.95
Animal
Cat
Dog
Pig

40. Naïve Bayesian Classifier
Problem!
Naïve Bayes assumes independence of features…
p(d|cj)
Naïve Bayesian Classifier
p(d1|cj) p(d2|cj) p(dn|cj)
Sex
Over 6 foot
Male
Yes
0.15 No
0.85
Female
0.01
0.99
Sex
Over 200 pounds
Male
Yes
0.11 No
0.80
Female
0.05
0.95

41. Naïve Bayesian Classifier
Solution
Consider the relationships between attributes…
p(d|cj)
Naïve Bayesian Classifier
p(d1|cj) p(d2|cj) p(dn|cj)
Sex
Over 6 foot
Male
Yes
0.15 No
0.85
Female
0.01
0.99
Sex
Over 200 pounds
Male
Yes and Over 6 foot
0.11
No and Over 6 foot
0.59
Yes and NOT Over 6 foot
0.05
No and NOT Over 6 foot
0.35
Female
0.01

42. Naïve Bayesian Classifier
Solution
Consider the relationships between attributes…
p(d|cj)
Naïve Bayesian Classifier
p(d1|cj) p(d2|cj) p(dn|cj)
But how do we find the set of connecting arcs??

43. The Naïve Bayesian Classifier has a quadratic decision boundary10 1 2 3 4 5 6 7 8 9

44. Dear SIR,
I am Mr. John Coleman and my sister is Miss Rose Colemen, we are the children of late Chief Paul Colemen from Sierra Leone. I am writing you in absolute confidence primarily to seek your assistance to transfer our cash of twenty one Million Dollars ($21, ) now in the custody of a private Security trust firm in Europe the money is in trunk boxes deposited and declared as family valuables by my late father as a matter of fact the company does not know the content as money, although my father made them to under stand that the boxes belongs to his foreign partner. …

45. This mail is probably spam
This mail is probably spam. The original message has been attached along with this report, so you can recognize or block similar unwanted mail in future. See for more details.
Content analysis details: (12.20 points, 5 required)
NIGERIAN_SUBJECT2 (1.4 points) Subject is indicative of a Nigerian spam
FROM_ENDS_IN_NUMS (0.7 points) From: ends in numbers
MIME_BOUND_MANY_HEX (2.9 points) Spam tool pattern in MIME boundary
URGENT_BIZ (2.7 points) BODY: Contains urgent matter
US_DOLLARS_ (1.5 points) BODY: Nigerian scam key phrase ($NN,NNN,NNN.NN)
DEAR_SOMETHING (1.8 points) BODY: Contains 'Dear (something)'
BAYES_ (1.6 points) BODY: Bayesian classifier says spam probability is 30 to 40%
[score: ]

46. Advantages/Disadvantages of Naïve Bayes
Fast to train (single scan). Fast to classify
Not sensitive to irrelevant features
Handles real and discrete data
Handles streaming data well
Disadvantages:
Assumes independence of features

47. Summary of Classification
We have seen 4 major classification techniques:
Simple linear classifier, Nearest neighbor, Decision tree.
There are other techniques:
Neural Networks, Support Vector Machines, Genetic algorithms..
In general, there is no one best classifier for all problems. You have to consider what you hope to achieve, and the data itself…
Let us now move on to the other classic problem of data mining and machine learning, Clustering…