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Recursive Feature Elimination: What It Is and Why It Matters?

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26th Mar, 2023
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Recursive Feature Elimination: What It Is and Why It Matters?

Data is the backbone of modern decision-making, and businesses are always looking for ways to extract valuable insights from it. Machine learning is one of the most common techniques deployed in organisations for data analysis, which involves training algorithms to make predictions based on historical data. However, not all features in a dataset are created equal, and some may have a higher impact on the model’s performance than others. 

Recursive feature elimination is a popular data analysis technique used to identify and eliminate irrelevant or redundant features from a dataset, improving the accuracy and efficiency of the machine learning model. 

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In this article, we will explore what recursive feature elimination is, how it works, and why it matters for businesses looking to extract meaningful insights from their data.

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What are the different techniques for feature selection?

Feature selection is a crucial step in machine learning that involves selecting the most relevant attributes from a dataset to build a model that accurately predicts outcomes. However, selecting the right features is not always straightforward. There are many different techniques, each with its strengths and weaknesses. Let’s take a look at some of them!

Filter Methods

Filter methods select features created on statistical properties, such as their correlation with the target variable or variance. These methods are computationally efficient and can be applied before training the model. Examples of filter methods include the Chi-squared test, Correlation-based feature selection, and variance thresholding.

Wrapper Methods

Wrapper methods select features by evaluating a machine-learning model’s performance with a subset of features. These methods are computationally expensive but can lead to better model performance. Examples of wrapper methods include Recursive Feature Elimination, Forward Selection, and Backward Elimination.

Embedded Methods

For embedded methods, feature selection occurs during training. These methods include techniques like Lasso and Ridge Regression, which add penalties to the model coefficients to shrink the less significant features to zero.

Hybrid Methods

Hybrid methods combine different feature selection techniques to achieve better results. These methods are often more effective than using a single approach alone. Examples of hybrid methods include ReliefF and Random Forest Feature Selection.

In essence, the choice of feature selection technique depends on the specific problem, dataset, and computational resources available. 

Now, let’s dive deeper into one of the most crucial wrapper methods for feature elimination, Recursive Feature Elimination. 

What Is Recursive Feature Elimination?

Recursive Feature Elimination (RFE) is a wrapper method that recursively eliminates features and builds a model over the remaining ones. It ranks the features based on importance and eliminates the least important ones until the desired number of features is reached. RFE is an iterative process that works as follows:

  1. Train the model on all the features and rank them based on their importance.
  2. Eliminate the least important feature.
  3. Repeatedly train the model on the remaining features and eliminate the least significant feature until the desired number of features is reached.

RFE considers the interaction between features and their impact on the model’s performance.

To understand how RFE works, let’s consider an example.

Suppose we have a dataset of housing prices with ten different features, including the number of bedrooms, square footage, and the age of the house. We want to build a machine-learning model to predict the price of a house based on these features. However, we suspect that some of the features may not be important and could even harm the model’s performance.

We can use RFE to identify the most relevant features by training the model with all the features and then recursively eliminating the least important ones until we reach the optimal subset. RFE trains the model during each iteration and evaluates its performance using a cross-validation set. 

For example, RFE may determine that the number of bedrooms, square footage, and location are the most critical features for predicting house prices. In contrast, other features, such as the age of the house, have little impact on the model’s accuracy.

Why did RFE come into the picture? What does it solve?

As machine learning became more prevalent, data scientists realised that some features might be irrelevant or redundant while others may significantly impact the model’s accuracy. This gave birth to one of the essential methods for building efficient machine-learning models- The feature Selection technique of Recursive Feature Elimination.

Recursive Feature Elimination (RFE) was introduced to address some of the limitations of existing methods while emerging as a wrapper method that recursively removes features and evaluates their impact on the model’s performance. The process continues until the optimal number of features is reached.

RFE solves several problems that traditional feature selection techniques encounter. 

  • RFE is a backward selection approach that starts with all features and then removes the least important ones iteratively. This approach is superior to forward selection, which starts with the least important feature and adds more until the optimal number is reached. 
  • RFE avoids overfitting by cross-validation during the feature selection process. Overfitting occurs when a model is too complex and fits the training data too well, resulting in poor performance on new data. 
  • RFE can be applied to any model type, making it a versatile technique that can be used in many different scenarios. 

Implementing the RFE algorithm in Python

Python provides several libraries that can be used for implementing the RFE algorithm. Let’s now take a look at a few RFE Python examples. 

RFE With scikit-learn

Scikit-learn is a popular machine-learning library in Python that provides a simple implementation of the RFE algorithm. The following code snippet demonstrates how to implement RFE in sci-kit-learn:

from sklearn.feature_selection import RFE

from sklearn.linear_model import LogisticRegression

model = LogisticRegression()

rfe = RFE(model, n_features_to_select=5)

rfe.fit(X, y)

In the code snippet above, we first import the RFE class from the feature_selection module of sci-kit-learn. We then create an instance of the LogisticRegression class, which will act as our base estimator. We then create an instance of the RFE class, passing the base estimator and the number of features to select. We then fit the RFE object to our data and labels.

RFE for Classification

In classification problems, RFE recursively removes features and builds a model on the remaining features. The feature ranking is based on the feature importance scores computed by the estimator. The following code snippet demonstrates using RFE for a classification problem:

from sklearn.datasets import make_classification

from sklearn.feature_selection import RFE

from sklearn.tree import DecisionTreeClassifier

X, y = make_classification(n_samples=1000, n_features=10, n_informative=5, n_redundant=0, random_state=42)

model = DecisionTreeClassifier()

rfe = RFE(model, n_features_to_select=5)

rfe.fit(X, y)

print(“Selected Features: “, rfe.support_)

print(“Feature Ranking: “, rfe.ranking_)

In the code snippet above, we first generate a synthetic dataset using the make_classification function from sci-kit-learn. We then create an instance of the DecisionTreeClassifier class, which will act as our base estimator. We then create an instance of the RFE class, passing the base estimator and the number of features to select. We then fit the RFE object into our data and labels, printing the chosen features and ranking features.

RFE Hyperparameters

RFE has several hyperparameters that can be tuned for better results. Some important hyperparameters are:

  • n_features_to_select: This hyperparameter determines the number of features to select.
  • step: This hyperparameter determines the number of features to remove each iteration. The default value is 1, which means one feature is removed at each iteration.
  • estimator: This hyperparameter specifies the base estimator to use. By default, a linear SVM is used.
  • scoring: This hyperparameter specifies the metric to use for feature ranking. The default value is None, meaning the estimator’s score method is used.
  • cv: This hyperparameter determines the cross-validation strategy to use. The default value is None, meaning a 3-fold cross-validation is used.

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Future of Recursive Feature Elimination

The future of Recursive Feature Elimination (RFE) looks promising, as it continues to be a popular technique for feature selection in machine learning. With the increasing amount of data being generated and the need for more efficient and accurate models, feature selection is becoming an essential step in the machine-learning pipeline.

Recent studies have shown that RFE can significantly improve the performance of machine learning models by reducing the dimensionality of the data and eliminating irrelevant or redundant features. For example, in a study by NCBI , RFE was used for feature selection in classifying depression patients based on functional magnetic resonance imaging (fMRI) data. The results showed that RFE selected a subset of features highly correlated with the clinical diagnosis of depression.

As the field of machine learning continues to grow, there is a need for more sophisticated and efficient feature selection techniques. One area of research that is gaining traction is the use of deep learning for feature selection. However, deep learning models are often computationally expensive and require training large data. 

In contrast, RFE is a simple and effective technique that can be applied to various models and datasets. Therefore, it is likely that RFE will continue to be used as a popular feature selection technique.

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Conclusion

In conclusion, Recursive Feature Elimination (RFE) is an effective technique for feature selection in machine learning which oversees a bright future following its evolving implementation. RFE, being an effective feature selection technique, is fueling its usage across diverse domains, such as medical diagnosis, bioinformatics, and image analysis, adding to its indomitable expansion.

If you want to learn more about machine learning and AI, consider enrolling in upGrad’s Machine Learning and AI PG Diploma program in collaboration with IIIT Bangalore. This comprehensive program covers the latest tools and techniques in machine learning and AI, including feature selection techniques like RFE. 

This program will give you the skills and knowledge needed to build and deploy machine-learning models for real-world applications. 

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Pavan Vadapalli

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Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.
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Frequently Asked Questions (FAQs)

1What is the difference between RFE and PCA for feature selection?

Both RFE and Principal Component Analysis (PCA) are techniques used for feature selection. The key difference between the two is that PCA modifies the original attributes into a fresh set, while RFE eliminates the original attributes.

2How do I determine the optimal number of features to select using RFE?

One way to determine the optimal number of features to select using RFE is to perform cross-validation and choose the number of features that gives the best performance on the validation set. Another way is to use a scree plot, which plots the number of features against the corresponding model performance.

3Can RFE be used for unsupervised learning tasks?

No, RFE is a supervised learning technique requiring labelled data to select features. Other techniques like clustering or dimensionality reduction may be used for feature selection in unsupervised learning tasks with no labelled data.

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