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In the world of machine learning, not all data is created equal — and understanding how to measure its uncertainty is key. That’s where entropy comes in. Whether you’re building a decision tree or analyzing dataset purity, entropy helps quantify how mixed or unpredictable your data really is. Let’s explore what entropy means in machine learning and how it drives better decisions in model training.

What is Entropy in Machine Learning?

In the world of machine learning and data science, entropy is a key concept used to measure the impurity or randomness in a dataset. It plays a fundamental role in algorithms like Decision Trees, helping them decide how to split data to build accurate predictive models.

Whether you’re a beginner or revisiting the concept, this guide will help you understand what entropy is in machine learning, how it’s calculated, and where it’s applied—with practical examples.

Understanding Entropy: The Basics

Entropy is a measure from information theory that quantifies the uncertainty or disorder in a dataset. Introduced by Claude Shannon, it helps machine learning models evaluate how mixed or pure the data is at any point.

In simple terms:

  1. Low entropy = data is more pure (less mixed).
  2. High entropy = data is more impure (more mixed).

Why is Entropy Important in Machine Learning?

In supervised learning, especially classification tasks, entropy helps models determine how informative a feature is for predicting a label. It’s most commonly used in Decision Tree algorithms like ID3, C4.5, and CART.

During tree construction, the algorithm selects the attribute with the lowest entropy (or highest information gain) to split the data.

How is Entropy Calculated?

The formula for entropy (H) is:

H(S)=−∑i=1npilog⁡2piH(S) = -\sum_{i=1}^{n} p_i \log_2 p_i

Where:

  1. SS is the dataset
  2. pip_i is the proportion of class ii
  3. nn is the total number of classes

Python Example: Entropy Calculation

Let’s calculate the entropy for a dataset with 9 positive and 5 negative examples.

import math

def entropy(p, n):

total = p + n

p_ratio = p / total

n_ratio = n / total

return -p_ratio * math.log2(p_ratio) - n_ratio * math.log2(n_ratio)

print("Entropy:", entropy(9, 5))

Output:

Entropy: 0.9402859586706309

This value tells us the current level of disorder in our dataset. A value close to 1 means the dataset is highly mixed.

Entropy in Decision Trees

Let’s say we’re building a tree to classify whether a customer will buy a product. One of the features is “Age” and we want to know whether splitting the dataset by age reduces entropy.

We calculate the entropy of the parent node, then calculate the weighted average entropy of child nodes after a split. The Information Gain is:

Information Gain=Entropy(parent)−Weighted Entropy(children)\text{Information Gain} = \text{Entropy(parent)} - \text{Weighted Entropy(children)}

The attribute with the highest Information Gain is selected for the split.

Real-world Use Case

In customer churn prediction, entropy helps identify which attributes (e.g., contract type, monthly charges) most clearly differentiate between customers who stay and those who leave. The clearer the split, the lower the entropy.

Entropy vs Gini Index

Metric Entropy Gini Index
Formula −∑pilog⁡2pi-\sum p_i \log_2 p_i 1−∑pi21 – \sum p_i^2
Interpretation Measures impurity Measures impurity
Speed Slower (uses log) Faster (no log)
Use in ID3, C4.5 CART

Both are used to measure impurity and decide splits in decision trees. While entropy gives a more information-theoretic view, Gini is computationally cheaper.

Struggling With ML Concepts Like Entropy?

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Conclusion

So, what is entropy in machine learning? It’s a mathematical tool used to measure disorder in data—especially valuable for decision-making processes like building decision trees. Understanding entropy helps you grasp how models decide splits and how they aim to reduce uncertainty at every step.

Whether you’re fine-tuning a classifier or learning how decision trees work, mastering entropy gives you a strong edge in building better models.

About Author

Jayanti Katariya is the CEO of Moon Technolabs, a fast-growing IT solutions provider, with 18+ years of experience in the industry. Passionate about developing creative apps from a young age, he pursued an engineering degree to further this interest. Under his leadership, Moon Technolabs has helped numerous brands establish their online presence and he has also launched an invoicing software that assists businesses to streamline their financial operations.

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