Random Forest | TrendSpider Learning Center (2024)

15 mins read

Random forest is a widely-used machine learning algorithm developed by Leo Breiman and Adele Cutler, known for its ability to combine the outputs of multiple decision trees to produce a single result. This algorithm is highly favored for its versatility and ease of use, handling both classification and regression tasks effectively.

To understand random forests, it is helpful to start with the basics of decision trees. A decision tree begins with a root node, which poses a question (e.g., “Should I surf today?”). Subsequent questions, known as decision nodes, split the data based on specific criteria (e.g., “Is it a long period swell?”). These splits guide the path through the tree until a final decision is reached at a leaf node.

Random forests enhance the predictive power of individual decision trees by employing ensemble learning, where multiple trees are generated and aggregated to produce a more accurate and robust model. The process involves two key components: Bagging (bootstrap aggregating) (Breiman, 1996) and the Classification and Regression Tree (CART) split (Breiman et al., 1984).

Bagging, introduced by Breiman in 1996, is a technique that improves the stability and accuracy of machine learning algorithms. It involves generating multiple subsets of the original dataset through random sampling with replacement. Each subset is used to train a separate decision tree, resulting in a diverse collection of trees. The final prediction is made by averaging the predictions of all individual trees (for regression) or by majority voting (for classification). This method reduces variance and helps prevent overfitting, making the model more robust to noise and variations in the data (Breiman, 1996).

The CART algorithm, involves recursively splitting the data into subsets based on feature values that result in the most significant reduction in impurity. For classification tasks, measures like Gini impurity or information gain are used to determine the best split, while for regression tasks, mean squared error (MSE) is typically used. Each decision node in the tree represents a feature and a threshold value, guiding the data down different branches until a prediction is made at the leaf nodes. By combining multiple CART trees in a random forest, the model benefits from the strengths of each individual tree while mitigating their weaknesses (Breiman et al., 1984).

When these two techniques are combined in a random forest, the result is a powerful ensemble model that leverages the advantages of both methods. Bagging ensures that each tree is trained on a unique subset of the data, promoting diversity among the trees. The CART algorithm optimally splits the data at each node, ensuring that each tree makes accurate and meaningful decisions. Together, they create a model that is more accurate, less prone to overfitting, and capable of handling a wide variety of data types and structures.

While single-decision trees are prone to bias and overfitting, random forests mitigate these issues by creating an ensemble of uncorrelated trees, leading to more accurate and reliable predictions. This methodology is particularly effective for large, high-dimensional datasets, making it a powerful tool for various applications.

The random forest methodology has been successfully involved in various practical problems as illustrated in the table below.

Origin & History

I. Early Development and Proposals

The concept of random decision forests was first introduced by Salzberg and Heath in 1993. They developed a method that utilized a randomized decision tree algorithm to generate multiple distinct trees and combined their outputs using majority voting. This foundational idea set the stage for further advancements in ensemble learning methods.

In 1995, Ho expanded upon this idea by demonstrating that forests of trees splitting with oblique hyperplanes could improve accuracy as they grew without suffering from overtraining. He found that as long as the forests were randomly restricted to be sensitive to only selected feature dimensions, the model’s performance improved.

Ho’s subsequent work concluded that other splitting methods behaved similarly, provided they were also forced to be insensitive to some feature dimensions. This observation was significant because it contradicted the common belief that increasing classifier complexity beyond a certain point would lead to overfitting. The robustness of the forest method against overtraining was further explained through Kleinberg’s theory of stochastic discrimination.

II. Influence of Prior Work

The early development of Breiman’s notion of random forests was significantly influenced by the work of Amit and Geman. They introduced the idea of searching over a random subset of the available decisions when splitting a node, specifically in the context of growing a single tree.

Another influential concept was random subspace selection, introduced by Ho. This method involved growing a forest of trees where variation among the trees was introduced by projecting the training data into a randomly chosen subspace before fitting each tree or each node.

Additionally, the idea of randomized node optimization, where the decision at each node is determined by a randomized procedure rather than a deterministic optimization, was first introduced by Thomas G. Dietterich.

III. Breiman’s Contribution

The formal introduction of random forests was made by Leo Breiman in a landmark paper. In this paper, Breiman described a method of constructing a forest of uncorrelated trees using a CART (Classification and Regression Tree) like procedure, combined with randomized node optimization and bagging (bootstrap aggregating).

Breiman’s paper brought together several components, some previously known and some novel, which became the foundation for modern random forest methods. Key contributions from this paper included:

• Using Out-of-Bag Error: Breiman introduced the concept of using out-of-bag error as an estimate of the generalization error, which provided a reliable measure of the model’s performance.

• Measuring Variable Importance: The paper proposed measuring variable importance through permutation. This method assesses the impact of each variable on the model’s accuracy by randomly permuting the feature values and observing the resulting changes in performance.

• Theoretical Insights: Breiman offered the first theoretical results for random forests, including a bound on the generalization error that depended on the strength of the trees in the forest and their correlation. This provided a theoretical foundation for understanding the robustness and accuracy of random forests.

These advancements collectively established random forests as a powerful and versatile machine learning technique, capable of handling complex classification and regression tasks with high accuracy and robustness.

Random Forest Architecture

The architecture of the Random Forest algorithm is structured around the concept of ensemble learning, where multiple decision trees are constructed and their outputs combined to produce a more accurate and robust model. This approach leverages the strengths of individual decision trees while mitigating their weaknesses. The diagram provided illustrates this process in detail.

1. Input Data (X)

The process begins with the input data, denoted as 𝑋. This dataset contains multiple features that will be used to train the individual decision trees within the random forest.

2. Construction of Multiple Decision Trees

In the Random Forest algorithm, a multitude of decision trees (𝑡𝑟𝑒𝑒1, 𝑡𝑟𝑒𝑒2,…,𝑡𝑟𝑒𝑒𝐵) are constructed. The key steps involved in this construction include:

• Bootstrapping (Bagging): From the original dataset, multiple subsets are created by randomly sampling with replacement. Each subset is used to train an individual decision tree. This technique is known as bootstrapping or bagging (Breiman, 1996).

• Feature Selection: During the training of each tree, a random subset of features is selected at each split to determine the best split. This random selection of features helps in reducing the correlation between the individual trees, making them more independent.

3. Decision Trees (𝑡𝑟𝑒𝑒1, 𝑡𝑟𝑒𝑒2,…,𝑡𝑟𝑒𝑒𝐵)

Each decision tree in the forest is constructed independently using the bootstrapped datasets and the selected random subset of features. The trees are built using the Classification and Regression Tree (CART) algorithm (Breiman et al., 1984), where the best splits are determined based on criteria like Gini impurity (for classification) or mean squared error (for regression).

4. Outputs of Individual Trees (𝑘1, 𝑘2,…,𝑘𝐵)

Once the trees are constructed, they are used to make predictions. Each tree produces an output (𝑘1, 𝑘2,…,𝑘𝐵) based on the input data 𝑋

5. Aggregation of Outputs

The final step in the Random Forest algorithm involves aggregating the outputs from all the individual trees to produce a single result. The method of aggregation depends on the type of task:

Classification: For classification tasks, a majority voting mechanism is used. Each tree votes for a class, and the class with the majority votes is selected as the final prediction.

Regression: For regression tasks, the outputs from all trees are averaged to produce the final prediction. This averaging helps in smoothing out the predictions and reducing variance.

Advantages of the Architecture

• Reduction of Overfitting: By aggregating the results of multiple trees, the random forest reduces the risk of overfitting, which is a common problem with individual decision trees.

• Improved Accuracy: The ensemble approach enhances the overall predictive accuracy of the model as it combines the strengths of multiple trees.

• Robustness to Noise: The random selection of features and bootstrapping make the random forest robust to noise and variability in the dataset.

Types of Random Forest

Random Forest is a versatile machine learning algorithm that has evolved into several variations to suit different data types and specific problem domains. Here are some key types and variations of the Random Forest algorithm:

I. Classification Random Forest

The most common type of Random Forest used for classification tasks. It involves creating multiple decision trees and combining their outputs to determine the class label for a given input. It is commonly used in image recognition, spam detection, and medical diagnosis. Example: Predicting whether an email is spam or not based on its content.

II. Regression Random Forest

Used for regression tasks where the goal is to predict a continuous output. Instead of class labels, the average of the outputs from individual trees is used to make predictions. Often used in financial forecasting, real estate pricing, and environmental modeling. Example: Predicting house prices based on various features like location, size, and number of rooms.

III. Quantile Regression Forests

An extension of the regression random forest that estimates conditional quantiles, providing a way to understand the distribution of the predicted values. Useful in financial risk management and weather forecasting. Example: Estimating the range within which a stock price might vary, giving both the central tendency and the spread.

IV. Survival Random Forests

Designed to handle censored data typically used in survival analysis. It is used to predict the time until an event of interest occurs. Widely used in medical research for patient survival time prediction and reliability engineering. Example: Predicting the survival time of patients based on their medical history and treatment plans.

V. Unsupervised Random Forests

Used for tasks like clustering and anomaly detection where there are no predefined labels. It works by analyzing the similarity between the data points. Suitable for market basket analysis, fraud detection, and outlier detection. Example: Identifying fraudulent transactions based on patterns that differ from normal transactions.

VI. Random Survival Forests (RSF)

A type of Random Forest specifically tailored for survival analysis, dealing with censored data and providing survival probabilities over time. Used in clinical studies to understand the impact of different variables on patient survival rates. Example: Assessing the survival probability of patients with different cancer treatments over time.

VII. Multivariate Random Forests

Capable of handling multiple outputs simultaneously, making them suitable for tasks where several dependent variables need to be predicted. Used in environmental science for predicting multiple weather parameters or in bioinformatics for multi-gene expression analysis. Example: Predicting temperature, humidity, and wind speed simultaneously in weather forecasting.

Advantages of Random Forest

Random Forest is a widely used ensemble learning method that offers several key advantages, making it a popular choice for both classification and regression tasks.

I. Handling Large Set

Random Forest is a highly effective, computationally intensive technique for improving unstable estimates, particularly in large, high-dimensional datasets where single-step model identification is impractical due to complexity and scale (Bühlmann and Yu, 2002; Kleiner et al., 2014; Wager et al., 2014). Its parallelizable nature allows it to handle extensive real-life systems efficiently. The R package randomForest, available on CRAN, and the MapReduce (Jeffrey and Sanja, 2008) implementation called Partial Decision Forests, accessible on the Apache Mahout website, enable the construction of forests with large datasets, provided each partition fits into memory.

II. Reduced Overfitting

Random forests reduce the risk of overfitting, which is a common issue with decision trees that tend to fit the training data too closely. By incorporating a large number of decision trees, random forests mitigate overfitting because the averaging of uncorrelated trees decreases overall variance and prediction error. This ensemble approach ensures that the model generalizes better to unseen data, leading to more accurate and reliable predictions.

III. Provides flexibility

Random forests offer significant flexibility, as they can effectively handle both regression and classification tasks with high accuracy, making them a favored choice among data scientists. Additionally, the feature bagging technique used in random forests makes this classifier adept at estimating missing values, ensuring the model maintains accuracy even when some data is missing. This versatility and robustness in dealing with various data challenges contribute to the widespread adoption and effectiveness of random forests in diverse applications.

IV. Evaluating Variable Importance

Random forests facilitate the evaluation of variable importance or the contribution of each feature to the model. This can be assessed through several methods. Gini importance and Mean Decrease in Impurity (MDI) are commonly used to determine how much the model’s accuracy decreases when a specific variable is excluded. Another method, permutation importance or Mean Decrease Accuracy (MDA), measures the average decrease in accuracy by randomly permuting the feature values in out-of-bag (OOB) samples. This provides a robust way to understand the significance of each feature in predicting the target variable.

Disadvantages of Random Forest

Despite its many strengths, Random Forest also has some limitations and challenges. Understanding these drawbacks is essential for making informed decisions about when and how to use this powerful algorithm.

I. Time-Consuming Process

Random forest algorithms are capable of handling large datasets and providing accurate predictions. However, this capability comes at a cost: the process can be slow. Each decision tree in the forest must be computed individually, leading to longer processing times compared to simpler algorithms. This is particularly noticeable when dealing with very large datasets or when the number of trees in the forest is high.

II. Resource-Intensive

Due to their ability to process and analyze extensive datasets, random forests require substantial computational resources. This includes not only processing power but also memory to store the data and intermediate results for each decision tree. The need for significant resources can be a limitation, especially for organizations with limited computational infrastructure.

III. Increased Complexity

While the prediction of a single decision tree is relatively straightforward to interpret, a random forest—comprising multiple decision trees—introduces a higher level of complexity. Interpreting the results from a forest of trees can be challenging because it involves understanding the aggregate output of numerous individual trees, each contributing to the final prediction. This complexity can make it harder to understand the model’s decision-making process and to communicate the results to stakeholders who may not have a technical background.

Constructing a Random Forest Model

Constructing a Random Forest model involves data preparation, training individual decision trees using bootstrapping and random feature selection, aggregating the predictions, and evaluating the model’s performance. By following these steps and using tools like Scikit-Learn, you can build robust and accurate Random Forest models for both classification and regression tasks.

I. Data Preparation

• Collect Data: Gather the dataset that you will use to train the Random Forest model. This dataset should include both the features (independent variables) and the target variable (dependent variable).

• Clean Data: Perform data cleaning to handle missing values, remove duplicates, and correct any inconsistencies in the data.

• Split Data: Divide the dataset into training and testing sets. A common split is 70% for training and 30% for testing.

II. Model Training

• Bootstrapping: For each decision tree, use a technique called bootstrap aggregating (bagging) to randomly sample the training data with replacement. This means each tree is trained on a different subset of the data, introducing further diversity among the trees. Some samples may be repeated, while others may be left out, known as “out-of-bag” (OOB) samples.

• Feature Selection: For each decision tree in the forest, randomly select a subset of the input features to use for splitting the nodes. This helps in creating diverse trees and reduces correlation among them. By default, the number of features selected is typically the square root of the total number of features for classification tasks and one-third of the total number of features for regression tasks.

III. Constructing Individual Decision Trees

• Training Decision Trees: For each subset of the training data, construct a decision tree using the selected subset of features. The decision tree is built by recursively splitting the data based on the feature that provides the best split according to criteria like Gini impurity (for classification) or mean squared error (for regression).

• Stopping Criteria: Set stopping criteria for tree growth, such as maximum tree depth, minimum samples per leaf, or maximum number of leaves to prevent overfitting.

IV. Aggregation of Trees

Once the forest of decision trees is constructed, make predictions on new data by aggregating the predictions from all individual trees. For regression tasks, average the results of all trees to get the final prediction. For classification tasks, use majority voting, where each tree votes for a class, and the class with the most votes is chosen as the final prediction.

V. Evaluate Model Performance

Assess the model’s performance using appropriate metrics. For regression tasks, common metrics include mean squared error (MSE) or mean absolute error (MAE). For classification tasks, metrics such as accuracy, precision, recall, and F1-score are used. The “out-of-bag” (OOB) samples, which were not used in training a particular tree, can be employed to get an unbiased estimate of the model’s generalization error.

VI. Tune Hyperparameters

Adjust the hyperparameters of the Random Forest model to optimize its performance. Important hyperparameters include:

• n_estimators: The number of decision trees in the forest.
  
• max_depth: The maximum depth of each decision tree.
  
• max_features: The number of features to consider when looking for the best split at each node.

Tuning these hyperparameters can significantly impact the model’s performance. Start with the default values and iteratively optimize them based on evaluation metrics to achieve the best results for your specific dataset and problem.

Example

Here’s an example of how to construct a Random Forest model using Python and the Scikit-Learn library:

Random Forest in Trading

Predicting stock market trends is notoriously challenging due to the inherent volatility and myriad factors influencing price movements. Traditional methods such as time series analysis and econometric models often fall short in capturing the complexities of the market. Recently, machine learning techniques, particularly ensemble learning methods like Random Forest (RF), have shown promise in addressing these challenges by treating stock price prediction as a classification problem.

Case Study I

The paper “Predicting the direction of stock market prices using random forest” by Luckyson Khaidem, Snehanshu Saha, and Sudeepa Roy Dey explores the use of the Random Forest algorithm to predict stock market trends. This method capitalizes on the ability of Random Forests to manage high-dimensional and complex data, making it suitable for the chaotic nature of stock price movements.

I. Data Collection & Preprocessing

The authors collected historical stock price data and applied exponential smoothing to reduce noise and highlight trends. This smoothing technique assigns exponentially decreasing weights to past observations, making recent data more influential. Technical indicators such as the Relative Strength Index (RSI), stochastic oscillator, and moving average convergence divergence (MACD) were then extracted from the smoothed data to form the feature set.

II. Feature Extraction

Technical indicators serve as the primary features for the Random Forest model. These indicators provide insights into potential future price movements and are commonly used in technical analysis.

III. Random Forest Algorithm

The RF algorithm constructs an ensemble of decision trees, each trained on a bootstrapped sample of the data. At each node, the best split is chosen based on a subset of randomly selected features, ensuring that the trees are uncorrelated and reducing the risk of overfitting. The final prediction is made by aggregating the predictions of all trees in the forest.

IV. Results

The Random Forest model was evaluated using several metrics, including accuracy, precision, recall, and specificity. These metrics were calculated for three datasets: Apple (AAPL), Microsoft (MSFT), and Samsung Electronics Co. Ltd. The results demonstrated that the RF model outperformed existing models in terms of accuracy and robustness.

• AAPL: Achieved an accuracy range of 85-95% for long-term predictions.
  
• MSFT: Demonstrated similar high accuracy rates, confirming the model’s robustness.
  
• Samsung: Consistent performance, validating the model’s effectiveness across different stocks.

ROC curves were plotted to further evaluate the model’s performance, graphically demonstrating the robustness of the predictions. Additionally, the model showed a significant decrease in Out-of-Bag (OOB) error rates as the number of trees in the forest increased, indicating convergence and stability.

Case Study II

The study, “Classification of Intraday S&P500 Returns with a Random Forest” by Lohrmann and Luukka (2019), addresses the challenge of predicting stock market movements, particularly intraday returns of the S&P500 index.

The primary objective of the study is to use feature selection and a random forest ensemble classifier to build a model for predicting the S&P500 open-to-close returns in a four-class setting. The study aims to analyze feature importance, develop trading strategies based on these predictions, and benchmark their performance against a buy-and-hold strategy.

Methodology

The methodology involves several steps:

• Feature Selection: The study uses entropy measures and the Fuzzy Similarity and Entropy Measure (FSAE) based feature selection algorithm to determine the most relevant features for classification. This process includes adding noise to the training data to enhance the robustness of the feature selection.

• Random Forest Classifier: The selected features are used to train a random forest classifier consisting of 50 decision trees. The classifier’s performance is compared with other classifiers like k-nearest neighbors (KNN), naive Bayes, and decision trees.

• Trading Strategies: Based on the classifier’s predictions, four trading strategies are developed. These strategies are evaluated against a buy-and-hold strategy for the test and forecast periods.

Results

The feature selection process identified the most informative features, including S&P500 momentum terms, currency exchange rates, the European stock market, and the United States Commodity Index (USCI). The random forest classifier with a minimum leaf size of 10 achieved the highest accuracy of 44.72% on the test set and 41.0% on the forecast data set.

The study also examined the performances of different trading strategies based on the classifier’s predictions. Strategy 4, which initiates trades based on the predictions of Classes 1 and 2 (positive returns) and Class 4 (negative returns), outperformed the buy-and-hold strategy with an average annual return of 21.09% after transaction costs, compared to 12.93% for the buy-and-hold strategy.

Case Study III

The study “Stock prediction based on random forest and LSTM neural network” presents a hybrid model combining RF and LSTM neural networks for stock price prediction, showing improved performance over traditional models. The proposed stock prediction model combines Random Forest (RF) and Long Short-Term Memory (LSTM) neural networks to enhance the accuracy of stock price forecasts. The model consists of two main components:

• Random Forest: RF is used to analyze the importance of input features and select the most relevant ones for prediction. This step involves fitting the training data to the RF model, obtaining importance scores for each feature, and selecting features with high importance scores.

• LSTM Neural Network: The selected features are then used to train an LSTM neural network. LSTM is a type of recurrent neural network designed to capture long-term dependencies in time series data, addressing the gradient disappearance problem and improving prediction accuracy (Hochreiter & Schmidhuber, 1997).

Data and Methodology

The study utilizes the Shanghai Composite Index (SCI) data from January 4, 2013, to November 30, 2017, as the experimental dataset. The dataset includes 35 technical indicators commonly used in stock market analysis. The data preprocessing steps involve standardizing the technical indicators and using RF to select the most important features.

The training process for the LSTM model follows a window marking approach, using the past 30 days’ data to predict the stock price trend for the next day. The dataset is divided into training (70%), validation (15%), and test (15%) sets to prevent overfitting.

Experimental Results

The experimental results demonstrate that the RF+LSTM model outperforms both the PCA+LSTM and LSTM models. Key performance metrics include accuracy, F-measure, and rate of return. The RF+LSTM model achieved an accuracy of 61.18%, an F-measure of 0.7589, and a rate of return of 5.90%, significantly higher than the benchmarks (Ma et al., 2019). The trading simulation results also indicate that the RF+LSTM model provides higher returns and a better information ratio compared to the other models and the buy-and-hold strategy.

The Bottom Line

Random Forest stands out as a robust and versatile machine learning algorithm, adept at handling both classification and regression tasks with high accuracy and resilience against overfitting. By leveraging the power of ensemble learning, it combines multiple decision trees to improve predictive performance and provide valuable insights into feature importance. This adaptability makes Random Forest an excellent choice for a wide range of applications, from image recognition and medical diagnosis to financial forecasting and environmental modeling. In the trading domain, Random Forest proves particularly valuable. It can effectively analyze vast amounts of market data, capturing complex, nonlinear relationships that are often missed by simpler models.