Feature Selection in Machine Learning

by | Sep 19, 2020 | Feature Selection, Machine Learning

When building a machine learning model for a business problem, it’s rare that all the variables in the data will need to be incorporated into the model. Sure, adding more variables rarely makes a predictive model less accurate, but there are certain disadvantages to including an excess of features.

It is commonly said that data scientists spend a huge amount of time doing data preprocessing, feature engineering, and feature extraction. Feature selection is also a very common step in many data science projects, and we usually test more than one feature selection method to find the best subset from the original features.

In this article, I discuss the importance of selecting features in machine learning. I highlight why we should select features when using our models for business problems. And then go over the main feature selection algorithms.

What we’ll cover:

  • What is feature selection in machine learning?
  • Importance of feature selection in machine learning
  • Feature selection methods: filter, wrapper, embedded and hybrid

Let’s get started.

For tutorials and step by step code implementations on additional feature selection methods, check out our course Feature Selection for Machine Learning or our book Feature Selection in Machine Learning with Python.

What is feature selection in machine learning?

Feature selection is the process of identifying and selecting a subset of variables from the original data set to use as inputs in a machine learning model.

A data set usually contains a large number of features. We can employ a variety of methods to determine which of these are actually relevant features when making predictions.

Each of the different methods have advantages and disadvantages to consider. But why should we select features to begin with?

Animation showing the process of feature selection

Importance of feature selection in machine learning

At a glance, it may seem that the more information one feeds to the machine learning model, the more accurate it will be. However, simpler models are more effective in a variety of ways, particularly when used in an organization.

Machine learning models with fewer features are:

  • Easier to interpret
  • Less redundant (have reduced feature redundancy)
  • Faster to train
  • Easier to deploy to production

 

Interpretability

If a model has 10 predictive variables, instead of a hundred, people can understand the effect of the variables on the outcome much more clearly.

Being able to interpret the result of a machine learning model is important when the model is used in real life, say for example to prevent fraud. When an observation is flagged as potentially fraudulent by a machine learning classifier, the fraud investigators will investigate it further. It helps them, if there are a few, clear indicators, the variables in the model, that point them in the right direction.

 

Redundancy

Oftentimes many of the variables in our datasets are redundant. Redundant variables won’t add information to the machine learning model. In fact, in some cases, large feature subsets may reduce the performance of some machine learning models. In particularly, those derived from tree-based algorithms.

Simpler models could generalize better and therefore show reduced overfitting. Too many variables often add noise to the model rather than predictive value. This is often referred to as the curse of dimensionality. Eliminating noisy and irrelevant features could make the model generalize better to new, unseen data, which gives way for more accurate predictions.

Speed

Additionally, simpler models require shorter training times. Training a machine learning model using less variables, reduce the computational cost. This speeds up model training and also allows the models to score the incoming data much faster. If a business intends to use the model to make real time decisions, speed is particularly important.

 

Streamlined deployment

Simpler models require simpler production code. For a model with less features, we need less error handling code and less unit testing. This makes model deployment easier to implement.

Models that use less features also have reduced the risk of data dependent errors; if a business uses data collected from a third party API, the more variables in the model, the more exposure they have to any errors or, simply, changes made by the third party, or even worse, a temporary shut-down of the API.

So selecting feature when using machine learning models in business, is important. How can we then select the most predictive features?

Animation showing the process of variable examination

With the following feature selection techniques, an organization can benefit from simpler models while still achieving effective and robust machine learning models.

How do we select features?

A feature selection procedure combines a search technique with an evaluation method. The search technique proposes new feature subsets, and the evaluation measure determines the how good the subset is.

In a perfect world, a feature selection method would evaluate all possible subsets of feature combinations and determine which one results in the best performing regression model or classifier.

However, computational cost inhibits such a practice in reality. In addition, the optimal subset of features varies between machine learning models. A feature subset that optimizes one model’s performance won’t necessarily optimize another’s.

To combat these challenges, there are many feature selection algorithms we can use to find a close-to-optimal subset of features in a manner that is computationally cost efficient.

Feature Selection in Machine Learning with Python, book cover

Generally, these methods have characteristics that allow them to group into one of three categories: filter methods, wrapper methods, and embedded methods.

Feature selection strategies

Filter methods select features based on characteristics in the data. They look at each feature individually, or compare them to other features, and assess how important they are.

Wrapper methods examine all or almost all possible feature combinations to identify the optimal feature subset. Because of this, they are known as “greedy” algorithms.

Embedded methods describe selection procedures that occur alongside fitting the machine learning model. Combining these two processes allows them to consider the interaction between the model and the features. And they are less computationally costly than wrapper methods.

Hybrid methods are feature selection algorithms that contain some characteristics of wrapper, some of filter and some of embedded methods.

Now that I’ve introduced the categories of feature selection procedures, I’ll go into more detail about each, highlight the advantages and disadvantages.

Filter methods

A typical filter algorithm consists of two steps: it ranks features based on certain criteria and then chooses the highest-ranking features to train the machine learning models.

Filter methods are generally univariate, so they rank each feature independently of the rest. Because of this, the filter methods tend to ignore any interactions that occur between features. Thus, redundant variables will not necessarily be eliminated by filter methods.

However, some multivariate filter selection methods exist as well. These consider features in relations to others in the data set, making them naturally capable of handling redundant features. Their selection criteria scans for duplicated features and correlated features and provide simple but powerful methods to quickly remove redundant information.

The filter methods are basic, but essential. Though they don’t usually find the best feature subset, they are model agnostic and are often an easy first step to reduce the feature space without significant computational cost.

I’ll describe some of the popular filter methods here, as well as provide insight about when to use them.

Constant, quasi-constant, and duplicated features

The most basic and intuitive methods for feature selection consist of removing constant, quasi-constant, or duplicated features.

Constant features only show one value for all the observations in the data set. That is, they show absolutely no variability.

Quasi-constant features are similar; if most observations share the same value, then we’d label that feature quasi-constant. In practice, quasi-constant features typically refer to those variables where more than 95 to 99 percent of the observations show the same value.

Constant and quasi-constant features can be eliminated by fixing a variance threshold, and then removing those features whose variance is below that threshold. The open-source library sklearn has a dedicated transformer to do this: VarianceThreshold.

Feature-engine, also allows you to drop constant and quasi-constant features through the DropConstantFeatures() class.

Duplicated features, as the name indicates, are those that are in essence, identical. That is, for every observation, they show the same value.

Removing constant and duplicated features can reduce the feature space dramatically.

Animation showing the distribution of a constant feature

Although it sounds obvious and overly simple, many datasets contain a lot of constant, quasi-constant, and duplicated features. In fact, duplicated features often arise when generating new features by one-hot encoding of categorical variables. Removing these features is an easy but effective way to reduce the dimension of the feature space, without losing any significant information.

Correlation

Correlation measures the association between two or more variables. The Pearson correlation coefficient is one measure of association, that determines the linear association between 2 continuous variables. There are other coefficients, like Kendall’s tau or Spearman’s that quantify non-linear associations.

The higher the correlation, the more linearly associated the variables are. The central hypothesis is that good feature sets contain features that are highly correlated with the target, yet uncorrelated with each other.

If two variables are correlated, we can predict one from the other. Therefore, if two features are correlated, the model only really needs one of them, as the second one does not add additional information.

Diagrams showing two correlated variables.

Though correlated features shouldn’t diminish the accuracy of the model, there are certain detriments to having them. For linear models, like linear regression or logistic regression, multi-collinearity may reduce performance or mask the true contribution of each feature to the prediction.

For decision trees and random forests, the model building methods will assign roughly the same importance for both correlated features, but this would be roughly half the importance it would assign if there were just one feature present, affecting the model interpretability.

Statistical measures

The filter methods discussed so far evaluate features individually or compare features to other features. Statistical methods for feature selection, evaluate features in light of the target. In short, the compare the distribution of the features when the target is of class 0 vs 1.

Diagram showing the process of feature selection by ranking features

Statistical tests compare the distribution of the features when the target is of class 0 with that shown when the target is of class 1. Then, it assigns a statistical value or p-value depending on how different the distributions are.

With this value, it ranks the features. And then the top ranking features are selected.

Mutual information

Mutual information measures the mutual dependence between two variables, in this case, the feature and the target. Mutual information is similar to correlation, but more general; it doesn’t strictly represent linear association. It measures how much knowing one of these variables reduces uncertainty in the other.

 

Chi-square

The chi-square test uses the chi-Square distribution to measure the dependency between two variables. It is suitable when evaluating categorical features against a binary or multi-class target variable.

The chi-square test essentially compares the actual frequencies of the values of a variable with the expected frequencies if it had no relation to the target.

For example, we can think of Titanic survivors. If there was no relationship between gender and survival, about 50 percent of the survivors would be male and 50 percent female.

However, it turns out about 75 percent of the survivors were female, suggesting that gender and survival have a significant relationship. Thus, the variable gender is very likely, a good predictor of the survival.

Contingency tables showing the absolute numbers and proportions of survivors in the titanic segregated by gender.

ANOVA

ANOVA is another method to measure dependencies between two variables, but unlike the chi-square test, is suited to continuous variables. It requires a binary target, and it essentially compares the distribution of the variable when the target is one versus the distribution when the target is zero.

 

Single feature models

ROC-AUC or RMSE measure the performance of a model for either classification or regression methods, respectively. We can select features by building a machine learning model using only one feature and then evaluating the model’s performance with one of these metrics. After repeating this for every feature, we can rank the features with this metric, then select the top-ranking ones.

Wrapper methods

In comparison to filter methods, wrapper methods tend to be more computationally expensive, but select a better set of features.

Wrapper methods use a specific machine learning algorithm in the selection process and choose the best subset of features tailored for that algorithm. This subset of features may not be optimal for a different machine learning model. In other words, wrapper methods are not model agnostic.

A typical wrapper method will perform the following:

Diagram showing the process of feature selection with wrapper methods

Wrapper methods start by searching through different subset of features, then creating a model with each. They evaluate these models to select the best one, and afterwards, they iterate to define a new subset based on the previous best subset.

Deciding when to stop this search comes down to monitoring whether the performance doesn’t increase or decrease beyond a certain threshold, depending on what method you’re using. These thresholds are often arbitrary and defined by the user.

I’ll discuss these procedures in more details for specific wrapper methods, including forward feature selection, backward feature selection, and exhaustive search.

Forward selection

Forward feature selection algorithm begins by evaluating all feature subsets that consist of only one input variable. It selects the “best” feature and afterwards, adds all the other features to it, individually, and selects a second feature that creates the new best performing model.

The process repeats over and over, adding one feature at a time, until it meets certain criteria. After adding additional features to the subset, if the machine learning model performance doesn’t improve by more than a specific threshold, then we can stop the search and select this feature subset.

Diagram showing the process of forward selection of features.

Backward selection

Backward feature selection works in an opposite way. It begins by building a machine learning model with all of the features. Then creates new subsets by removing every feature, one at a time, and training a new machine learning model.

It evaluates each subset to find the best one, then continues the process of removing a new single feature that results in the best model.

Diagram showing the process of backward elimination of features.

The process stops when an additional feature is removed and the performance doesn’t decrease past a certain arbitrary threshold.

An exhaustive search is the most “greedy” method. It tries all combinations of features, for any number of features from one until the maximum number of features available.

This method is extremely computationally expensive, almost prohibitively, but would provide the best subset of features, at least in theory.

For example, if there were four features to choose from (A, B, C, and D), an exhaustive search would build and evaluate models for fifteen feature subsets.

It would go through all possible groupings: each feature on its own, every combination of paired features, every threesome combination, and the set of all features, as depicted below.

Table showing all possible feature combinations created from a data set with 4 features: A, B, C and D.

Embedded methods

Finally, we have embedded methods. These encompass the benefits of both the wrapper and filter methods, by evaluating interactions of features but also maintaining reasonable computational cost.

The typical steps for embedded methods involve training a machine learning algorithm using all the features, then deriving the importance of those features according to the algorithm used.

Afterwards, it can remove unimportant features based on some criteria specific to the algorithm, of which I’ll cover some examples of shortly.

Lasso

The Lasso regularization adds a penalty to different coefficients to reduce their freedom, thereby reducing its tendency to fit the noise. The higher the penalty, the less the model will overfit and the better it will generalize to unseen data.

During the Lasso fitting algorithm, the model tries to minimize the difference between the predicted and estimated value of the observation with the penalty.

Lasso can shrink some coefficients of the linear regression to zero. This indicates that the predictor can essentially be multiplied by zero to estimate the target and consequently doesn’t add to the overall prediction of the output.

In this way, Lasso regularization helps determine which features can be removed from the model.

Regression coefficients

Linear models aim to predict a target based on given variables by assigning a coefficient to each. They follow the form of this equation:

Linear model equation

Here, each variable X is multiplied by a coefficient, beta, to predict the final value of y. The betas here are directly proportional to how much the feature contributes to y. This holds true for any regularized or unregularized linear methods. Consequently, we are able to use these coefficients to select features.

There are a few caveats, however: this method assumes there is a linear relation between the predictors and the target and the scale of the features affects interpretability of the coefficients.

Some features may only contain values between zero and one, while others may range into the thousands or greater, which would affect the size of the coefficient. To truly compare features, we’d need to put all the features into similar scales.

If all of these conditions hold and the features are in a similar scale, we can safely infer that the coefficients align with variable importance and therefore remove any that have negligible coefficients.

Decision tree feature importance

Random forests and decision trees are generally very popular among the machine learning algorithms because they provide good predictive performance, low overfitting, and easy interpretability.

Part of the interpretability encompasses how straightforward it is to derive the importance of each feature on a decision, which makes it one of the best embedded methods of feature selection.

Random forests are a collection of decision trees. Each decision tree is comprised of a series of “questions” based on different features in the data set.

For each feature, the decision tree formulates a question of the type “is the value of the observation bigger than a for feature x?”

If the answer is yes, the observation is allocated on one side of the node, if it is no, the data is taken to the other side of the node.

The answer leads to the highest possible decrease of impurity, or in other words, the maximum information gain, meaning it gives the best possible separation of the class.

Diagram of a decision tree

Random forests consist of hundreds to thousands of decision trees built over a random extraction of the observations and features from the data set. The random extraction helps train de-correlated trees, and thus improve generalization.

Each node of the decision tree separates the data into two buckets. The importance of the feature is derived by combining the height of the node in which the feature is used, with how many different trees use the feature, and by how much the impurity is decreased.

We can therefore select features by building a random forest, looking at the importance derived for each feature, and selecting the most important ones.

Hybrid methods

As the name might suggest, hybrid methods combine pieces of wrapper methods and embedded methods. They build several algorithms at each round of selection like wrapper methods, but they don’t examine every possible feature combination.

In addition, hybrid feature selection algorithms select features based on a specific machine learning model and evaluate the model performance.

Some common hybrid methods are recursive feature elimination and recursive feature addition.

Recursive feature elimination

Recursive feature elimination (RFE) consists of the following steps:

Diagram showing the process of recursive feature elimination

It starts with ranking the feature with the importance derived from an embedded method, such as those discussed earlier.

Next, we remove the least important feature and build a new machine learning algorithm. We then calculate a performance metric, such as ROC-AUC, MSE, or RMSE.

If the metric decreases by more than an arbitrarily set threshold, then the feature should be kept. Otherwise, we can remove that feature and repeat this process until a feature removal causes the performance metric to decrease past this threshold.

The difference between this method and the step backward feature selection method is that here, we don’t remove all features first in order to determine which to remove. We simply select the least important feature based on what the machine learning model derives as important. It removes only one feature, rather than all the features at each step.

Because of this, in recursive feature elimination we train significantly less number of models compared to step backward feature selection.

Recursive feature addition

The recursive feature addition algorithm works similarly, but adding features one at a time rather than removing one.

Diagram showing the process of recursive feature addition.

It begins by ranking the features with one of the embedded methods, then builds a model with only the one most important feature.

Again, we calculate the performance metric of our choice. Then we add the second most important feature and train a model. And if the metric increases over an arbitrarily set threshold, then we should keep the feature. Otherwise, it can be removed.

This process will repeat until all features have been evaluated. Similar to recursive elimination, it differs from step forward feature selection because it doesn’t add all features to determine what to keep, but instead, just adds the most important one.

This method and the recursive feature elimination method usually work faster than wrapper methods and better than embedded methods. They account for correlations depending on how stringent the threshold is set, but on the downside, this threshold is, again, set arbitrarily; the smaller the threshold, the more features will be selected.

Random shuffling of the feature values

Another popular method of feature selection consists in randomly shuffling the values of a specific variable and observing how the permutation affects the performance metric of the algorithm.

This is a good alternative to embedded methods, for those artificial intelligence algorithms that do not derive feature importance during the induction, like for example, nearest neighbors or support vector machines (svm). However, this is also commonly used with linear and decision tree based models.

If the variable is important, the random permutation will decrease the performance metrics dramatically. On the other hand, permuting unimportant variables should have little to no effect on the performance of the model. Thus, this information can be used to determine which variables are predictive enough to keep in the feature subset.

Animation showing the process of permutation feature importance

Feature selection vs dimensionality reduction

Very often, students of my course “Feature Selection for machine learning model”, ask me why I do not cover PCA in the course. The thing is, principal component analysis (PCA) is not a feature selection method. It is a dimensionality reduction procedure. With PCA, we combine the existing features into principal components, and then we select a subset of them. But we still need all the features to create the few selected PA. So it does not really reduce the amount of effort needed to deploy a model to production.

In short, in feature selection, we select a subset of features from the data set to train machine learning algorithms. They do not alter the original features distribution. Dimensionality reduction methods, like PCA, alter the original representation of the variables.

Feature selection and deep learning

Many students ask me if the methods that I teach in my course are also suitable for deep learning. The thing is, deep learning models give us an advantage over traditional off-the-shelf algorithms only when we have high-dimensional data with lots of observations and features. Only in these cases will neural networks return models that outperform traditional machine learning algorithms.

Selecting features is commonly done when training traditional machine learning models. Hence, all the methods that have been developed and discussed in the literature are suitable only for these methods.

Wrapping up

Feature selection in machine learning models allows for accurate models that are simpler and less computationally expensive to implement.

The methods I’ve outlined in this blog post should serve as a guide for selecting features to optimize a model in a variety of different situations.

Though there isn’t one perfect method, each method provides advantages and disadvantages, and between all the methods available, one should be able to find one to optimize the machine learning model.

 

References

Feature Selection for Machine Learning, online course.