# 5. Consider the following data set for a binary class problem. A B Class Label T F T T T T T F − T T F F − F F − F F − T T − T F − a. Calculate the information gain when splitting on A and B.

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5. Consider the following data set for a binary class problem.

A B Class Label

T F

T T

T T

T F −

T T

F F −

F F −

F F −

T T −

T F −

a. Calculate the information gain when splitting on A and B. Which

attribute would the decision tree induction algorithm choose?

b. Calculate the gain in the Gini index when splitting on A and B.

Which attribute would the decision tree induction algorithm

choose?

c. Figure 3.11 shows that entropy and the Gini index are both

monotonically increasing on the range [0, 0.5] and they are both

monotonically decreasing on the range [0.5, 1]. Is it possible that

information gain and the gain in the Gini index favor different

attributes? Explain.

7. Consider the following set of training examples.

X Y Z No. of Class C1 Examples No. of Class C2 Examples

0 0 0 5 40

0 0 1 0 15

0 1 0 10 5

0 1 1 45 0

1 0 0 10 5

1 0 1 25 0

1 1 0 5 20

1 1 1 0 15

a. Compute a two-level decision tree using the greedy approach

described in this chapter. Use the classification error rate as the

criterion for splitting. What is the overall error rate of the induced

tree?

b. Repeat part (a) using X as the first splitting attribute and then

choose the best remaining attribute for splitting at each of the two

successor nodes. What is the error rate of the induced tree?

c. Compare the results of parts (a) and (b). Comment on the suitability

of the greedy heuristic used for splitting attribute selection.

8. The following table summarizes a data set with three attributes A, B,

C and two class labels , −. Build a two-level decision tree.

A B C

Number of Instances

−

T T T 5 0

F T T 0 20

T F T 20 0

F F T 0 5

T T F 0 0

F T F 25 0

T F F 0 0

F F F 0 25

a. According to the classification error rate, which attribute would be

chosen as the first splitting attribute? For each attribute, show the

contingency table and the gains in classification error rate.

b. Repeat for the two children of the root node.

c. How many instances are misclassified by the resulting decision

tree?

d. Repeat parts (a), (b), and (c) using C as the splitting attribute.

e. Use the results in parts (c) and (d) to conclude about the greedy

nature of the decision tree induction algorithm.

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