Understanding Bootstrap Aggregation and Random Forest

December 17, 2023

In the world of machine learning, there are numerous techniques and algorithms that empower predictive modeling and data analysis. Two such powerful methods are Bootstrap Aggregation, commonly known as Bagging, and Random Forest. These techniques are widely used for their robustness and ability to improve the accuracy and stability of machine learning models.

2013 12 17

What is Bootstrap Aggregation (Bagging)?

Bootstrap Aggregation, or Bagging, is an ensemble learning technique used to improve the stability and accuracy of machine learning algorithms. It reduces variance and helps to avoid overfitting. The concept of Bagging was introduced by Leo Breiman in 1994 and has since become a cornerstone in the field of machine learning.

How Does Bagging Work?

Bagging involves creating multiple versions of a predictor and using these to get an aggregated predictor. The main steps are:

  1. Random Sampling with Replacement: The original dataset is sampled randomly with replacement, creating multiple bootstrapped datasets.
  2. Model Training: A model is trained separately on each bootstrapped dataset.
  3. Aggregation of Predictions: The predictions from each model are combined (usually by averaging for regression problems or voting for classification problems) to form a final prediction.

The beauty of Bagging lies in its simplicity and effectiveness, especially for decision tree algorithms, where it significantly reduces variance without increasing bias.

Random Forest: An Extension of Bagging

Random Forest is a popular ensemble learning technique that builds upon the concept of Bagging. Developed also by Leo Breiman, it involves constructing a multitude of decision trees at training time and outputting the class that is the mode of the classes (classification) or mean prediction (regression) of the individual trees.

How Does Random Forest Differ from Basic Bagging?

  1. Use of Decision Trees: Random Forest specifically uses decision trees as its base learners.
  2. Feature Randomness: When building each tree, a random subset of features is chosen. This ensures that the trees are de-correlated and makes the model more robust to noise.
  3. Multiple Trees: A Random Forest typically involves a larger number of trees, providing a more accurate and stable prediction.

Advantages of Random Forest

  • High Accuracy: Random Forests often produce highly accurate models, especially for complex datasets.
  • Robust to Overfitting: Due to the averaging of multiple trees, the risk of overfitting is lower compared to individual decision trees.
  • Handles Large Datasets Efficiently: They are capable of handling large datasets with higher dimensionality.

Applications and Considerations

Both Bagging and Random Forest find applications in various fields, including finance for credit scoring, biology for gene classification, and many areas of research and development. However, while using these techniques, one must be mindful of the following:

  • Computational Complexity: Both methods can be computationally intensive, especially Random Forest with a large number of trees.
  • Interpretability: Decision trees are inherently interpretable, but when combined into a Random Forest, the interpretability decreases.
  • Parameter Tuning: Tuning parameters like the number of trees, depth of trees, and number of features considered at each split is crucial for optimal performance.


Bootstrap Aggregation and Random Forest are powerful techniques in the arsenal of a data scientist. By understanding and correctly applying these methods, one can significantly improve the performance of machine learning models, tackling both bias and variance, and thereby making robust and accurate predictions. As with any tool, their effectiveness depends largely on the skill and understanding of the practitioner in applying them to the right kind of problems.

Profile picture

Victor Leung, who blog about business, technology and personal development. Happy to connect on LinkedIn