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# 203.3.3 How Decision tree Splits works?

### The Splitting Criterion

In previous section, we studied about The Decision Tree Approach

• The best split is
• The split that does the best job of separating the data into groups
• Where a single class(either 0 or 1) predominates in each group

## Impurity (Diversity) Measures

• We are looking for a impurity or diversity measure that will give high score for this Age variable(high impurity while segmenting), Low score for Gender variable(Low impurity while segmenting)
• Entropy: Characterizes the impurity/diversity of segment
• Measure of uncertainty/Impurity
• Entropy measures the information amount in a message
• S is a segment of training examples, p+ is the proportion of positive examples, p- is the proportion of negative examples
• Entropy(S) = $$-p_+ log_2p_+ – p_- log_2 p_-$$
• Where $$p_+$$ is the probabailty of positive class and $$p_-$$ is the probabailty of negative class
• Entropy is highest when the split has p of 0.5.
• Entropy is least when the split is pure .ie p of 1

### Entropy is highest when the split has p of 0.5

• Entropy(S) = $$-p_+ log_2p_+ – p_- log_2 p_-$$
• Entropy is highest when the split has p of 0.5
• 50-50 class ratio in a segment is really impure, hence entropy is high
• Entropy(S) = $$-p_+ log_2p_+ – p_- log_2 p_-$$
• Entropy(S) = $$-0.5*log_2(0.5) – 0.5*log_2(0.5)$$
• Entropy(S) = 1

### Entropy is least when the split is pure .ie p of 1

• Entropy(S) = $$-p_+ log_2p_+ – p_- log_2 p_-$$
• Entropy is least when the split is pure ie p of 1
• 100-0 class ratio in a segment is really pure, hence entropy is low
• Entropy(S) = $$-p_+ log_2p_+ – p_- log_2 p_-$$
• Entropy(S) = $$-1*log_2(1) – 0*log_2(0)$$
• Entropy(S) = 0

### The less the entropy, the better the split

• The less the entropy, the better the split
• Entropy is formulated in such a way that, its value will be high for impure segments

The next post is about How to Calculate Entropy for Decision Tree Split.

20th June 2017
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