Data Mining Lecture Series

1. Prof. Neeraj Bhargava
Vishal Dutt
Department of Computer Science, School of
Engineering & System Sciences
MDS University, Ajmer

2. 2
Attribute Selection Measure: Information
Gain (ID3/C4.5)
 S contains si tuples of class Ci for i = {1, …, m}
 information measures info (entropy) required to classify
any arbitrary tuple
 Assume a set of training examples, S. If we make attribute
A, with v values, the root of the current tree, this will
partition S into v subsets. The expected information
needed to complete the tree after making A the root is:
s
s
log
s
s
),...,s,ssI(
i
m
i
i
m21 2
1


)s,...,s(I
s
s...s
E(A) mjj
v
j
mjj
1
1
1




3. Information gain
 information gained by branching on
attribute A
 We will choose the attribute with the
highest information gain to branch the
current tree.
E(A))s,...,s,I(sGain(A) m  21
3

4. Attribute Selection by info gain
 Class P: buys_computer = “yes”
 Class N: buys_computer = “no”
 I(p, n) = I(9, 5) =0.940
 Compute the entropy for age:
means “age <=30” has 5
out of 14 samples, with 2
yes’es and 3 no’s. Hence
Similarly,
4
age pi ni I(pi, ni)
<=30 2 3 0.971
30…40 4 0 0
>40 3 2 0.971
694.0)2,3(
14
5
)0,4(
14
4
)3,2(
14
5
)(


I
IIageE
048.0)_(
151.0)(
029.0)(



ratingcreditGain
studentGain
incomeGain
246.0)(),()(  ageEnpIageGain
age income student credit_rating buys_computer
<=30 high no fair no
<=30 high no excellent no
31…40 high no fair yes
>40 medium no fair yes
>40 low yes fair yes
>40 low yes excellent no
31…40 low yes excellent yes
<=30 medium no fair no
<=30 low yes fair yes
>40 medium yes fair yes
<=30 medium yes excellent yes
31…40 medium no excellent yes
31…40 high yes fair yes
>40 medium no excellent no
)3,2(
14
5
I

5. We build the following tree
5
age?
overcast
student? credit rating?
no yes fairexcellent
<=30 >40
no noyes yes
yes
30..40

6. Extracting Classification Rules from Trees
 Represent the knowledge in the form of IF-THEN rules
 One rule is created for each path from the root to a leaf
 Each attribute-value pair along a path forms a conjunction.
The leaf node holds the class prediction
 Rules are easier for humans to understand
 Example
IF age = “<=30” AND student = “no” THEN buys_computer = “no”
IF age = “<=30” AND student = “yes” THEN buys_computer = “yes”
IF age = “31…40” THEN buys_computer = “yes”
IF age = “>40” AND credit_rating = “excellent” THEN buys_computer =
“yes”
IF age = “<=30” AND credit_rating = “fair” THEN buys_computer = “no”
6

7. Avoid Overfitting in Classification
 Overfitting: An tree may overfit the training data
 Good accuracy on training data but poor on test exmples
 Too many branches, some may reflect anomalies due to
noise or outliers
 Two approaches to avoid overfitting
 Prepruning: Halt tree construction early
 Difficult to decide
 Postpruning: Remove branches from a “fully grown”
tree—get a sequence of progressively pruned trees.
 This method is commonly used (based on validation set or
statistical estimate or MDL)
7

8. Enhancements to basic decision
tree induction
 Allow for continuous-valued attributes
 Dynamically define new discrete-valued attributes that
partition the continuous attribute value into a discrete set
of intervals
 Handle missing attribute values
 Assign the most common value of the attribute
 Assign probability to each of the possible values
 Attribute construction
 Create new attributes based on existing ones that are
sparsely represented.
 This reduces fragmentation, repetition, and replication 8