How Does Bootstrap Aggregation (Bagging) Work?

Machine learning processes known as "bagging" or bootstrap aggregation use ensemble learning to develop machine learning models. This strategy, which was developed in the 1990s, makes use of particular groups of training sets where some observations may be repeated throughout training sets.

Machine learning has considerably used the bagging concept to improve model fitting. The concept is that multiple separate machine learning units can work together more effectively than a single unit with more resources.

Consider each step in the bagging process as a separate brain to understand better how this works. Without bagging, machine learning would be a single extremely intelligent mind solving a problem. In the case of bagging, numerous "weak brains" or less capable brains work together on a job. Each has a distinct area of thought, some of which overlap. The final product is much more sophisticated when combined than it would be with just one "brain."

A very old maxim that predates technology by many years can be used to define the philosophy of bagging: "two heads are better than one." In bagging, ten, twenty, or fifty heads are preferable to one because the results are combined to get a superior outcome. Bagging is a method that engineers can use to combat the "overfitting" problem in machine learning, which occurs when the system does not suit the data or the objective.

How Bootstrap Aggregation Works

The bagging method, which Breiman Leo introduced in 1996, consists of three fundamental steps:


Bagging uses a bootstrapping sampling strategy to provide a variety of samples. This resampling technique creates several subsets of the training dataset by randomly and replacement-wise picking data points. This indicates that you have the option to select the same instance more than once each time you choose a data point from the training dataset. A value or instance is repeated twice (or more) in a sample.

Parallel Instruction:

These bootstrap samples are then trained separately and concurrently using a base or weak learners.


Finally, an average or a majority of the predictions are picked, depending on the job (such as regression or classification), to produce a more accurate estimate. Regression involves averaging every output predicted by each classifier; this process is referred to as soft voting. Hard voting, often known as majority voting, is the process of accepting the class that receives the most votes in classification issues.

Explore a few benefits and challenges of bagging

Advantages of Bagging

Implementation Simplicity

To enhance model performance, it is simple to integrate the predictions of base learners or estimators using Python libraries like scikit-learn (also known as sklearn). The accessible modules are laid out in their documentation, which you can use for your model optimization.

Lowering the Variance

A learning algorithm's variance can be decreased via bagging. This is especially useful for high-dimensional data, where the presence of missing values can increase variance, increase the risk of overfitting, and hinder proper generalization to new datasets.

The Main Difficulties in Bootstrap Aggregation

Interpretability Loss

Because of the averaging that occurs among forecasts, it is challenging to derive extremely precise business insights from bagging. A more accurate or full dataset could produce more accuracy inside a single classification or regression model, even though the output is more exact than any particular data point.

Costly in Terms of Computation

With more iterations, bagging slows and becomes more time-consuming. It is, therefore, inadequate for real-time applications. For fast-generating bagged ensembles on sizable test sets, clustered computers with a lot of processing cores are appropriate.

Less Adaptable

Bagging as a strategy is particularly effective with less reliable algorithms. Less variety exists within the dataset of the model, therefore, ones that are more stable or highly biased do not gain as much. For large enough b, "bagging a linear regression model will merely return the original predictions," as stated in the Hands-On Guide to Machine Learning.

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