Feature Engineering Techniques

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Feature Engineering is the process of transforming raw data into meaningful input features that help Machine Learning models perform better.

It is one of the MOST important skills in ML because:

• Better features → Better model accuracy

• Helps extract patterns hidden in the data

• Reduces noise, improves training efficiency

• Required for real-world dirty datasets

Types of Feature Engineering

Feature engineering broadly contains:

1. Handling Missing Values

2. Encoding Categorical Variables

3. Scaling & Normalization

4. Feature Creation (New Features)

5. Transformation of Variables

6. Feature Extraction

7. Dimensionality Reduction

8. Outlier Handling

9. Datetime Feature Engineering

10. Text Feature Engineering

11. Feature Selection Techniques

Now let’s explore each in detail ????

1. Handling Missing Values

✔ Mean/Median Imputation

df['age'].fillna(df['age'].mean(), inplace=True)

✔ Mode Imputation (categorical)

df['city'].fillna(df['city'].mode()[0], inplace=True)

✔ Constant Imputation

df.fillna("Unknown", inplace=True)

✔ Advanced: KNN Imputer

from sklearn.impute import KNNImputer
imputer = KNNImputer(n_neighbors=5)
df[['age','salary']] = imputer.fit_transform(df[['age','salary']])

2. Encoding Categorical Variables

✔ One-Hot Encoding

pd.get_dummies(df, columns=['gender'], drop_first=True)

✔ Label Encoding

from sklearn.preprocessing import LabelEncoder
le = LabelEncoder()
df['gender'] = le.fit_transform(df['gender'])

✔ Ordinal Encoding (for ordered categories)

df['education'] = df['education'].map({"High School":1, "Bachelor":2, "Master":3, "PhD":4})

✔ Target Encoding (advanced)

Replace categories with mean of target.

3. Scaling & Normalization

✔ Standardization (Z-Score)

from sklearn.preprocessing import StandardScaler
scaler = StandardScaler()
df[['age','salary']] = scaler.fit_transform(df[['age','salary']])

✔ Min-Max Scaling

from sklearn.preprocessing import MinMaxScaler
scaler = MinMaxScaler()
df[['price']] = scaler.fit_transform(df[['price']])

✔ Robust Scaling (good for outliers)

from sklearn.preprocessing import RobustScaler
rs = RobustScaler()
df[['income']] = rs.fit_transform(df[['income']])

4. Creating New Features

✔ Mathematical Features

df['bmi'] = df['weight'] / (df['height']/100)**2

✔ Interaction Features (Feature crossing)

df['income_per_age'] = df['income'] / df['age']
df['area'] = df['length'] * df['width']

✔ Polynomial Features

from sklearn.preprocessing import PolynomialFeatures
poly = PolynomialFeatures(degree=2)
poly_features = poly.fit_transform(df[['area']])

5. Transformation of Variables

✔ Log Transformation (for skewed data)

df['salary_log'] = np.log(df['salary'] + 1)

✔ Square Root

df['sqrt_amount'] = np.sqrt(df['amount'])

✔ Box-Cox Transformation

from scipy.stats import boxcox
df['box'], _ = boxcox(df['variable'] + 1)

6. Feature Extraction

✔ From Text using TF-IDF

from sklearn.feature_extraction.text import TfidfVectorizer
tfidf = TfidfVectorizer()
X = tfidf.fit_transform(df['review'])

✔ From Images (using deep learning)

• CNN features (ResNet, VGG, MobileNet)

• Pretrained embeddings

7. Dimensionality Reduction

✔ PCA (Principal Component Analysis)

from sklearn.decomposition import PCA
pca = PCA(n_components=2)
X_pca = pca.fit_transform(df)

✔ t-SNE (for visualization)

from sklearn.manifold import TSNE
tsne = TSNE(n_components=2)
X_tsne = tsne.fit_transform(df)

8. Handling Outliers

✔ IQR Method

Q1 = df['price'].quantile(0.25)
Q3 = df['price'].quantile(0.75)
IQR = Q3 - Q1

df = df[(df['price'] > Q1 - 1.5*IQR) & (df['price'] < Q3>1.5*IQR)]

✔ Capping Outliers

df['income'] = df['income'].clip(lower=df['income'].quantile(0.01),
                                 upper=df['income'].quantile(0.99))

9. Date-Time Feature Engineering

Assume column: date

df['date'] = pd.to_datetime(df['date'])

✔ Extract:

df['year'] = df['date'].dt.year
df['month'] = df['date'].dt.month
df['day'] = df['date'].dt.day
df['weekday'] = df['date'].dt.dayofweek
df['is_weekend'] = (df['weekday'] >= 5).astype(int)

✔ Time difference

df['days_since_signup'] = (pd.Timestamp.today() - df['signup_date']).dt.days

10. Text Feature Engineering

✔ Token Count

df['word_count'] = df['review'].apply(lambda x: len(x.split()))

✔ Sentiment Polarity

from textblob import TextBlob
df['sentiment'] = df['review'].apply(lambda x: TextBlob(x).sentiment.polarity)

✔ Remove stopwords

import re
import nltk
from nltk.corpus import stopwords
stop = set(stopwords.words('english'))
df['clean_text'] = df['review'].apply(lambda x: " ".join([w for w in x.split() if w.lower() not in stop]))

11. Feature Selection Techniques

✔ Filter Methods

from sklearn.feature_selection import SelectKBest, chi2
best = SelectKBest(score_func=chi2, k=5)
X_new = best.fit_transform(X, y)

✔ Wrapper Method (RFE)

from sklearn.feature_selection import RFE
from sklearn.linear_model import LogisticRegression
model = LogisticRegression()
rfe = RFE(model, 5)
fit = rfe.fit(X, y)

✔ Embedded Methods (Lasso)

from sklearn.linear_model import Lasso
lasso = Lasso(alpha=0.01)
lasso.fit(X, y)

End-to-End Feature Engineering Example

import pandas as pd
import numpy as np
from sklearn.preprocessing import StandardScaler, LabelEncoder

df = pd.read_csv("employees.csv")

# Missing values
df['salary'].fillna(df['salary'].mean(), inplace=True)

# Encoding
le = LabelEncoder()
df['gender'] = le.fit_transform(df['gender'])

# Date-time
df['hire_date'] = pd.to_datetime(df['hire_date'])
df['experience_years'] = (pd.Timestamp.today() - df['hire_date']).dt.days / 365

# Feature creation
df['income_per_age'] = df['salary'] / df['age']

# Scaling
scaler = StandardScaler()
df[['salary_scaled']] = scaler.fit_transform(df[['salary']])

print(df.head())

Conclusion

Feature Engineering is the heart of Machine Learning.

It improves:

✔ Model accuracy
✔ Data quality
✔ Training speed
✔ Pattern extraction

It includes:

• cleaning

• creating

• encoding

• scaling

• transforming

• selecting

• extracting

Good ML models are built not by algorithms but by strong features.