Decision Tree


Decision Trees are fundamental tools in machine learning and data analysis. They provide a structured way to make decisions based on multiple criteria, just like choices in our daily lives. In this comprehensive guide, we will delve into how decision trees work, explain their results, and provide Python code examples to solidify your understanding.

How Does it Work?

A decision tree is a tree-like graph that is used to make decisions or predictions. It breaks down a decision into a series of questions and answers, ultimately leading to a final decision or prediction. Let’s explore the key components of a decision tree and how it works with a practical example.

Components of a Decision Tree:

  1. Root Node: This is the topmost node of the tree, representing the initial decision or question. It splits into branches based on the possible answers.
  2. Decision Nodes: These nodes follow the root node and represent questions or decisions. Each decision node leads to one or more child nodes, branching based on the possible outcomes.
  3. Leaf Nodes: These are the endpoints of the tree and represent the final decision or prediction. They do not split further.

Example: Decision Tree for Loan Approval

Let’s consider a classic example of a decision tree for loan approval. Suppose a bank needs to decide whether to approve a loan application. The decision tree might look like this:

 Loan Application
                        /   |   \
                Credit Score  Income
                 /    |    \
          Good   Fair   Low
  • Starting at the root node, the bank first considers the applicant’s credit score.
  • If the credit score is “Good,” the decision tree leads to an approval.
  • If the credit score is “Fair,” the bank further considers the applicant’s income.
  • If the income is “High,” the decision tree leads to an approval.
  • If the income is “Low,” the decision tree results in a rejection.

This simplified decision tree demonstrates how a complex decision, such as loan approval, can be broken down into a series of simple questions, allowing for a systematic decision-making process.

Result Explained

Understanding the outcomes or results of a decision tree is crucial. Each leaf node represents a final decision or prediction. In classification problems, the decision tree might predict classes, while in regression, it predicts numerical values.

Example: Predicting Species of Iris Flowers

Suppose we have a dataset of iris flowers with features like sepal length, sepal width, petal length, and petal width. We want to predict the species of iris flowers using a decision tree.

After training the decision tree, it might look like this:

Iris Flower
                   /   |   \
          Setosa  Versicolor  Virginica
  • If the decision tree leads to the “Setosa” leaf node, it predicts that the iris flower belongs to the Setosa species.
  • If the decision tree leads to the “Versicolor” leaf node, it predicts Versicolor.
  • If the decision tree leads to the “Virginica” leaf node, it predicts Virginica.

By following the path from the root node to the appropriate leaf, we can make predictions based on the features of the iris flower.

Python Code Example:

# Importing the necessary libraries
from sklearn.datasets import load_iris
from sklearn.tree import DecisionTreeClassifier

# Loading the Iris dataset
iris = load_iris()
X =
y =

# Creating a Decision Tree classifier
clf = DecisionTreeClassifier(), y)

# Making predictions
sample_data = [[5.1, 3.5, 1.4, 0.2]]  # Example iris flower features
predicted_class = clf.predict(sample_data)
print(f"Predicted class: {iris.target_names[predicted_class][0]}")

In this code example, we import the necessary libraries, load the Iris dataset, create a Decision Tree classifier, and make predictions on a sample iris flower.


Mastering decision trees is essential for anyone entering the world of machine learning and data analysis. By understanding how decision trees work and interpreting their results, you can make informed decisions and predictions in various domains. Use Python as your tool of choice to implement and experiment with decision trees, expanding your data analysis and machine learning skills.