Understanding Random Forest Algorithm With Examples

Sruthi E R 04 Jul, 2024
11 min read


Random Forest is a widely-used machine learning algorithm developed by Leo Breiman and Adele Cutler, which combines the output of multiple decision trees to reach a single result. Its ease of use and flexibility have fueled its adoption, as it handles both classification and regression problems. In this article, we will understand how random forest algorithm works, and about its advantages , regression techniques and how it differs from other algorithms and how to use it.

Learning Objectives

  • Learn the working of random forest with an example.
  • Understand the impact of different hyperparameters.
  • Implement them on a classification problem using scikit-learn.

This article was published as a part of the Data Science Blogathon.

Assumptions of Random Forest

To effectively use Random Forest, it is important to understand the underlying assumptions of the algorithm:

  • Independence of Trees: The decision trees in the forest should be independent of each other. This is achieved through bootstrap sampling and feature randomness.
  • Sufficient Data: Random Forest requires a large amount of data to build diverse trees and achieve optimal performance.
  • Balanced Trees: The algorithm assumes that the individual trees are grown sufficiently deep to capture the underlying patterns in the data.
  • Noisy Data Handling: Random Forest can handle noisy data, but it assumes that the noise is randomly distributed and not systematic.

In this article, we will understand how the Random Forest algorithm works and discuss its advantages, regression techniques, differences from other algorithms, and practical usage.

What is Random forest?

Random forest, a popular machine learning algorithm developed by Leo Breiman and Adele Cutler, merges the outputs of numerous decision trees to produce a single outcome. Its popularity stems from its user-friendliness and versatility, making it suitable for both classification and regression tasks.

Its widespread popularity stems from its user-friendly nature and adaptability, enabling it to tackle both classification and regression problems effectively. The algorithm’s strength lies in its ability to handle complex datasets and mitigate overfitting, making it a valuable tool for various predictive tasks in machine learning.

One of the most important features of the Random Forest Algorithm is that it can handle the data set containing continuous variables, as in the case of regression, and categorical variables, as in the case of classification. It performs better for classification and regression tasks. In this tutorial, we will understand the working of random forest and implement random forest on a classification task.

Random Forest Applications

  • Customer churn prediction: Businesses can use random forests to predict which customers are likely to churn (cancel their service) so that they can take steps to retain them. For example, a telecom company might use a random forest model to identify customers who are using their phone less frequently or who have a history of late payments.
  • Fraud detection: Random forests can identify fraudulent transactions in real-time. For instance, a bank might employ a random forest model to spot transactions made from unusual locations or involving unusually large amounts of money.
  • Stock price prediction: Random forest can predict future stock prices. However, it is important to note that stock price prediction is a very difficult task, and no model is ever going to be perfectly accurate.
  • Medical diagnosis: These can help doctors diagnose diseases. For example, a doctor might use a random forest model to help them diagnose a patient with cancer.
  • Image recognition: It can recognize objects in images. For example, a self-driving car might use a random forest model to identify pedestrians and other vehicles on the road.

Real-Life Analogy of Random Forest

Let’s dive into a real-life analogy to understand this concept further. A student named X wants to choose a course after his 10+2, and he cant decide which course fit for his skill set. So he decides to consult various people like his cousins, teachers, parents, degree students, and working people. He asks them varied questions like why he should choose, job opportunities with that course, course fee, etc. Finally, after consulting various people about the course he decides to take the course suggested by most people.

random forest flowchart

Working of Random Forest Algorithm

Before understanding the working of the random forest algorithm in machine learning, we must look into the ensemble learning technique. Ensemble simplymeans combining multiple models. Thus a collection of models is used to make predictions rather than an individual model.

Ensemble uses two types of methods:

random forest | methods of ensemble

As mentioned earlier, Random forest Classifier works on the Bagging principle. Now let’s dive in and understand bagging in detail.


Bagging, also known as Bootstrap Aggregation, serves as the ensemble technique in the Random Forest algorithm. Here are the steps involved in Bagging:

  1. Selection of Subset: Bagging starts by choosing a random sample, or subset, from the entire dataset.
  2. Bootstrap Sampling: Each model is then created from these samples, called Bootstrap Samples, which are taken from the original data with replacement. This process is known as row sampling.
  3. Bootstrapping: The step of row sampling with replacement is referred to as bootstrapping.
  4. Independent Model Training: Each model is trained independently on its corresponding Bootstrap Sample. This training process generates results for each model.
  5. Majority Voting: The final output is determined by combining the results of all models through majority voting. The most commonly predicted outcome among the models is selected.
  6. Aggregation: This step, which involves combining all the results and generating the final output based on majority voting, is known as aggregation.
bagging random forest

Now let’s look at an example by breaking it down with the help of the following figure. Here the bootstrap sample is taken from actual data (Bootstrap sample 01, Bootstrap sample 02, and Bootstrap sample 03) with a replacement which means there is a high possibility that each sample won’t contain unique data. The model (Model 01, Model 02, and Model 03) obtained from this bootstrap sample is trained independently. Each model generates results as shown. Now the Happy emoji has a majority when compared to the Sad emoji. Thus based on majority voting final output is obtained as Happy emoji.

bagging ensemble model random forest


Boosting is one of the techniques that use the concept of ensemble learning. A boosting algorithm combines multiple simple models (also known as weak learners or base estimators) to generate the final output. It is done by building a model by using weak models in series.

There are several boosting algorithms; AdaBoost was the first really successful boosting algorithm that was developed for the purpose of binary classification. AdaBoost is an abbreviation for Adaptive Boosting and is a prevalent boosting technique that combines multiple “weak classifiers” into a single “strong classifier.” There are Other Boosting techniques. For more, you can visit – 4 Boosting Algorithms You Should Know: GBM, XGBoost, LightGBM & CatBoost.

Steps Involved in Random Forest Algorithm

  • Step 1: In the Random forest model, a subset of data points and a subset of features is selected for constructing each decision tree. Simply put, n random records and m features are taken from the data set having k number of records.
  • Step 2: Individual decision trees are constructed for each sample.
  • Step 3: Each decision tree will generate an output.
  • Step 4: Final output is considered based on Majority Voting or Averaging for Classification and regression, respectively.

For example:

Consider the fruit basket as the data as shown in the figure below. Now n number of samples are taken from the fruit basket, and an individual decision tree is constructed for each sample. Each decision tree will generate an output, as shown in the figure. The final output is considered based on majority voting. In the below figure, you can see that the majority decision tree gives output as an apple when compared to a banana, so the final output is taken as an apple.

instance random forest

Important Features of Random Forest

  • Random Forest is distinguished by several key features that contribute to its effectiveness and versatility:
  • Diversity: Each decision tree in the Random Forest is built from a different subset of data and features. This diversity helps in reducing overfitting and improving the model’s generalization capability.
  • Robustness: By averaging the results from multiple trees, Random Forest reduces the variance and improves the robustness of the predictions.
  • Handling of Missing Values: Random Forest can handle missing values internally by using surrogate splits or by averaging results from other trees that do not have missing values for the same data points.
  • Feature Importance: It provides insights into the importance of each feature in the prediction process. This can be particularly useful for feature selection and understanding the underlying data patterns.
  • Scalability: Random Forest can be parallelized because each tree is built independently of the others. This makes it scalable to large datasets and high-dimensional data.
  • Versatility: It can be used for both classification and regression tasks. The algorithm is also effective for tasks involving categorical and continuous variables.
  • Stability: Due to the ensemble nature, Random Forest is less sensitive to changes in the training data compared to a single decision tree.
  • Out-of-Bag Error Estimation: Random Forest provides an internal mechanism for estimating the model error without the need for a separate validation set. This is done using the out-of-bag (OOB) samples, which are not used in the construction of each tree.

Difference Between Decision Tree and Random Forest

Random forest is a collection of decision trees; still, there are a lot of differences in their behavior.

Decision treesRandom Forest
1. Decision trees normally suffer from the problem of overfitting if it’s allowed to grow without any control.1. Random forests are created from subsets of data, and the final output is based on average or majority ranking; hence the problem of overfitting is taken care of.
2. A single decision tree is faster in computation.2. It is comparatively slower.
3. When a data set with features is taken as input by a decision tree, it will formulate some rules to make predictions.3. Random forest randomly selects observations, builds a decision tree, and takes the average result. It doesn’t use any set of formulas.

Thus random forests are much more successful than decision trees only if the trees are diverse and acceptable.

Important Hyperparameters in Random Forest

Hyperparameters are used in random forests to either enhance the performance and predictive power of models or to make the model faster.

Increase the Predictive Power

  • n_estimators: Number of trees the algorithm builds before averaging the predictions.
  • max_features: Maximum number of features random forest considers splitting a node.
  • mini_sample_leaf: Determines the minimum number of leaves required to split an internal node.
  • criterion: How to split the node in each tree? (Entropy/Gini impurity/Log Loss)
  • max_leaf_nodes: Maximum leaf nodes in each tree

Increase the Speed

  • n_jobs: it tells the engine how many processors it is allowed to use. If the value is 1, it can use only one processor, but if the value is -1, there is no limit.
  • random_state: controls randomness of the sample. The model will always produce the same results if it has a definite value of random state and has been given the same hyperparameters and training data.
  • oob_score: OOB means out of the bag. It is a random forest cross-validation method. In this, one-third of the sample is not used to train the data; instead used to evaluate its performance. These samples are called out-of-bag samples.

Coding in Python – Random Forest Classifier

Now let’s implement Random Forest in scikit-learn.

1. Let’s import the libraries.

# Importing the required libraries
import pandas as pd, numpy as np
import matplotlib.pyplot as plt, seaborn as sns
%matplotlib inline

2. Import the dataset.

Python Code:

random forest import data

3. Putting Feature Variable to X and Target variable to y.

# Putting feature variable to X
X = df.drop('heart disease',axis=1)
# Putting response variable to y
y = df['heart disease']

4. Train-Test-Split is performed

# now lets split the data into train and test
from sklearn.model_selection import train_test_split

# Splitting the data into train and test
X_train, X_test, y_train, y_test = train_test_split(X, y, train_size=0.7, random_state=42)
X_train.shape, X_test.shape
train test split random forest

5. Let’s import RandomForestClassifier and fit the data.

from sklearn.ensemble import RandomForestClassifier

classifier_rf = RandomForestClassifier(random_state=42, n_jobs=-1, max_depth=5,
                                       n_estimators=100, oob_score=True)

classifier_rf.fit(X_train, y_train)
wall time random forest
# checking the oob score
oob score

6. Let’s do hyperparameter tuning for Random Forest using GridSearchCV and fit the data.

rf = RandomForestClassifier(random_state=42, n_jobs=-1)

params = {
    'max_depth': [2,3,5,10,20],
    'min_samples_leaf': [5,10,20,50,100,200],
    'n_estimators': [10,25,30,50,100,200]

from sklearn.model_selection import GridSearchCV

# Instantiate the grid search model
grid_search = GridSearchCV(estimator=rf,
                           cv = 4,
                           n_jobs=-1, verbose=1, scoring="accuracy")

grid_search.fit(X_train, y_train)
rf_best = grid_search.best_estimator_

From hyperparameter tuning, we can fetch the best estimator, as shown. The best set of parameters identified was max_depth=5, min_samples_leaf=10,n_estimators=10

7. Now, let’s visualize

from sklearn.tree import plot_tree
plot_tree(rf_best.estimators_[5], feature_names = X.columns,class_names=['Disease', "No Disease"],filled=True);
from sklearn.tree import plot_tree
plot_tree(rf_best.estimators_[7], feature_names = X.columns,class_names=['Disease', "No Disease"],filled=True);

The trees created by estimators_[5] and estimators_[7] are different. Thus we can say that each tree is independent of the other.

8. Now let’s sort the data with the help of feature importance

feature importance
imp_df = pd.DataFrame({
    "Varname": X_train.columns,
    "Imp": rf_best.feature_importances_

imp_df.sort_values(by="Imp", ascending=False)

Advantages and Disadvantages of Random Forest Algorithm


  • It can be used in classification and regression problems.
  • It solves the problem of overfitting as output is based on majority voting or averaging.
  • It performs well even if the data contains null/missing values.
  • Each decision tree created is independent of the other; thus, it shows the property of parallelization.
  • It is highly stable as the average answers given by a large number of trees are taken.
  • It maintains diversity as all the attributes are not considered while making each decision tree though it is not true in all cases.
  • It is immune to the curse of dimensionality. Since each tree does not consider all the attributes, feature space is reduced.
  • We don’t have to segregate data into train and test as there will always be 30% of the data, which is not seen by the decision tree made out of bootstrap.


  • Random forest is highly complex compared to decision trees, where decisions can be made by following the path of the tree.
  • Training time is more than other models due to its complexity. Whenever it has to make a prediction, each decision tree has to generate output for the given input data.


Random forest is a great choice if anyone wants to build the model fast and efficiently, as one of the best things about the random forest Classifier is it can handle missing values. It is one of the best techniques with high performance, widely used in various industries for its efficiency. It can handle binary, continuous, and categorical data. Overall, random forest is a fast, simple, flexible, and robust model with some limitations.

Key Takeaways

  • Random forest algorithm is an ensemble learning technique combining numerous classifiers to enhance a model’s performance.
  • Random Forest is a supervised machine-learning algorithm made up of decision trees.
  • It is used for both classification and regression problems.

Frequently Asked Questions

Q1. How do you explain a random forest?

A. Random forest is an ensemble learning method combining multiple decision trees, enhancing prediction accuracy, reducing overfitting, and providing insights into feature importance, widely used in classification and regression tasks.

Q2. How random forest works step by step?

A. Random forest works by first randomly selecting subsets of the training data. For each subset, it constructs decision trees, splitting the data at each node based on the best feature from a random subset of features. Each tree then makes a prediction for a given input. Finally, the random forest combines the predictions of all trees, using averaging for regression tasks or majority voting for classification tasks, to produce the final output.

Q3. What are the advantages of Random Forest?

A. Random Forest tends to have a low bias since it works on the concept of bagging. It works well even with a dataset with a large no. of features since it works on a subset of features. Moreover, it is faster to train as the trees are independent of each other, making the training process parallelizable.

Q4. Why do we use random forest algorithms?

A. Random forest algorithms are used for their superior prediction accuracy, ability to handle large datasets, versatility in tasks, robustness to noise, and capability to provide feature importance insights.

Q5. What is the difference between random forest and regression?

A. Random forest is an ensemble learning method that combines multiple decision trees to improve accuracy and reduce overfitting, used for both classification and regression tasks. Regression, on the other hand, is a statistical technique that models the relationship between dependent and independent variables to predict continuous outcomes.

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Sruthi E R 04 Jul, 2024

Frequently Asked Questions

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Responses From Readers


Joe 23 Jun, 2021

Great article. Quiet an interesting read. A good balance of content and visuals presented in an easy to understand manner. Kudos!

Jo 23 Jun, 2021

Good job! Interesting article, presented in a easy to understand manner. Good luck!

Jo 24 Jun, 2021

Great Article. Concise and clear. Gives a good understanding of the concept with some interesting visual as well. Kudos!

HemaVal 24 Jun, 2021

Good article. Easy to understand for newbies like me.

Tadiwanashe Gata
Tadiwanashe Gata 08 Dec, 2021

it is nice and it helped me a lot so I'm thankful for your support

Sweety Gupta
Sweety Gupta 07 Jan, 2022

really great article with well explained stuff...thanx Shruthi

Azadi 15 Jan, 2022

Hello and thank you for the your nice article and the material you have shared. Please note that the meaning of hyper parameters of max_features, min_sample_leaf and oob_score are most probably not exact and correct in this article.

Lara Marques
Lara Marques 20 Feb, 2022

I have just one question. If 30% is left out by the model, why did you use train_test_split, splitting 70% of the dataset to train the model? Thanks a lot for the article! It was very enlightening.

akash 08 Apr, 2022

The hyperparameter section : "mini_sample_leaf– determines the minimum number of leaves required to split an internal node." Couldn't find any hyperparameter as such.

Nini 28 Apr, 2022

Thank you for publishing this article, it's so useful. Real-world examples are even more helpful, I really enjoyed reading it.

Surendra 26 May, 2022

Very well explained. Thank you.

zarmeen 26 Sep, 2022

Well written and informative content. Why did you choose data science after doing civil engg?

Muhammad Aftab
Muhammad Aftab 23 Nov, 2022

Good work COMPRISED of all basic and introductory information. While reading i was thinking that both classification and regression will be covered. but at the end majority of the work was based upon classification. I need regression based work too. But no problem material is still important and good covered.

Rim 16 Feb, 2023

Can the author explain this in details: "Train-Test split: In a random forest, we don’t have to segregate the data for train and test as there will always be 30% of the data which is not seen by the decision tree." Thanks,

Richmond 24 Jul, 2023

All in all, great product out here and its communicated in a digestible language. Only one concern. if we do not need a train_test split, then why do we have that in the code?

Exoticaleathers 19 Aug, 2023

The blog titled "Understanding Random Forest" offers a clear and insightful exploration of the Random Forest algorithm, a powerful tool in machine learning. With a blend of in-depth explanations and practical examples, the blog breaks down the complexity of Random Forest into manageable concepts, making it accessible even to those new to the field.

Tilahun Mamiye Gidey
Tilahun Mamiye Gidey 21 Sep, 2023

Good work !!! Simple and clear explanation about Random Forest Algorithm.