What is the difference between Feature Engineering and Feature Selection?

Related questions:
– What are some common feature engineering techniques?
– What is Feature Standardization and why is it needed?

“A machine learning model is only as good as the data”.

In this context, Feature engineering plays a crucial role in achieving good performance for a predictive modeling problem.

Feature Engineering

Feature Engineering is the process of preparing data for modeling. It involves combining existing features, creating new features based on domain knowledge, or transforming features to make them more suitable for a particular algorithm. It is a crucial step in the machine learning pipeline, as the quality of the features used can greatly affect the accuracy and generalization of the model.

Broadly speaking, there are two steps to Feature Engineering: 1) Feature Identification, and b) Feature Transformation. 

1. Feature Identification: In this step, we use domain expertise to identify characteristics that we think might have a predictive effect on the outcome. For ex: If the goal is to predict the selling price of a house, then lot size, number of bedrooms, year of construction, installed appliances etc. can have an effect on the final housing price, and thus should be included as features when building a machine learning model. 
   
2. Feature Transformation on the other hand takes these identified features, and transforms them into numerical representation that can be used by a machine learning model. Here are some common techniques used in feature transformation:
   
   • Feature scaling: Some machine learning algorithms are sensitive to the scale of the features, so it is often useful to scale the features to a common range, such as [0, 1] or [-1, 1]. This can be done using techniques such as min-max scaling or standardization.
   • One-hot encoding: This technique is used to represent categorical features as binary vectors. Each category is represented by a binary vector where one element is set to 1 and the rest are set to 0. This allows the machine learning algorithm to better handle categorical features.
   • Feature creation: This involves creating new features using existing ones based on domain knowledge. For example, if we have a dataset of customer transactions, we can create new features such as the total amount spent by each customer, the number of purchases made by each customer, or the average amount spent per purchase.
   • Feature transformation: This involves transforming features to make them more suitable for a particular algorithm. For example, we may transform a skewed distribution to a more normal distribution using a logarithmic transformation or a Box-Cox transformation.

Once all these features are extracted, they are concatenated to form a vector, which is a numerical representation of the raw data, to be consumed by the machine learning models. In order to achieve the best predictive performance, it may be necessary to systematically test a variety of data representations. In nutshell, this process of creating representations of data that increase the effectiveness of a model is feature engineering.

Feature Selection

Feature Selection on the other hand is the process of identifying a subset of features that are most predictive of the outcome. The goal of feature selection is to improve the performance of a machine learning model by reducing the number of features that the model needs to consider while still preserving its predictive power.

There are several reasons why feature selection is important.

• First, it can improve the accuracy and efficiency of a model by reducing the number of irrelevant or redundant features that can introduce noise and complexity to the data. For example, a bigger lot size can be positively correlated with the number of bedrooms, and thus might mask the effect of the ‘number of bedrooms’ feature.
• Second, it can help to avoid overfitting, which occurs when a model is too complex and captures noise in the training data instead of the underlying patterns.
• Third, it can simplify the interpretation of the model by identifying the most important factors that contribute to its predictions.

There are several techniques for feature selection, including:

1. Filter methods: These methods use statistical measures such as correlation, chi-square test, mutual information, etc., to rank the features based on their relevance to the target variable. The features with the highest scores are selected.
2. Wrapper methods: These methods use a machine learning model to evaluate the performance of different subsets of features. The model is trained on each subset of features and its performance is evaluated using cross-validation. The subset of features that produces the best performance is selected. If there is a large number of features, the wrapper method may become computationally extensive.
3. Embedded methods: These methods perform feature selection as part of the model training process. The model is designed to automatically select the most important features while learning the patterns in the data. A few examples of embedded methods include Regularized regression such as Lasso, Ridge, and Elastic-net, Decision Tree based methods such as Random Forest, Support Vector Machines (SVM), and Neural Networks

The choice of feature selection method depends on the specific characteristics of the dataset and the goals of the analysis. In general, it is important to strike a balance between selecting enough features to capture the important patterns in the data, while avoiding overfitting and unnecessary complexity.