Data Preprocessing in Machine Learning: 7 Key Steps to Follow, Strategies, & Applications
Updated on Feb 21, 2025 | 20 min read | 159.5k views
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Updated on Feb 21, 2025 | 20 min read | 159.5k views
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Have you ever fed raw data into a machine learning model only to get inconsistent or inaccurate results? It’s frustrating, right? That’s because raw data is often messy, incomplete, and unreliable. Data preprocessing in machine learning is the secret to transforming that chaos into a clean, structured format your models can actually understand.
In fact, data scientists spend nearly 80% of their time cleaning and organizing data—because well-prepared data leads to reliable outcomes. Paired with feature engineering in machine learning, it allows you to create meaningful inputs that improve model performance.
This guide will walk you through practical steps for data preprocessing and feature engineering, helping you unlock the full potential of your machine-learning projects. Dive in!
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Data preprocessing in machine learning involves transforming raw, unorganized data into a structured format suitable for machine learning models. This step is essential because raw data often contains missing values, inconsistencies, redundancies, and noise.
Preprocessing addresses these issues, ensuring that data is accurate, clean, and ready for analysis.
Unstructured data, such as text or sensor data, presents additional challenges compared to structured datasets. This process plays a key role in feature engineering in machine learning by preparing the data for further transformations and optimizations.
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Challenges with Raw Data and Their Solutions
Working with raw data often presents challenges that can impact the accuracy and efficiency of machine learning models. Here’s how data preprocessing in machine learning tackles these issues:
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Addressing these challenges ensures a strong foundation for machine learning models and supports effective feature engineering. This leads to the critical tasks involved in preprocessing.
Data preprocessing consists of multiple steps that prepare data for machine learning. Each task plays a distinct role in refining data and making it suitable for algorithms. Let’s explore them one by one.
Data cleaning focuses on identifying and fixing inaccuracies or inconsistencies in raw data. This step ensures that your dataset is reliable and ready for analysis.
Properly cleaned data ensures that the next step, integrating various data sources, proceeds seamlessly.
Data integration combines information from different sources into a single, cohesive dataset. This is especially critical when working with data collected from multiple systems or platforms.
Also Read: Talend Data Integration Architecture & Functional Blocks
Integrated data requires transformation into standardized formats for better compatibility with machine learning models.
Data transformation prepares integrated data for machine learning by converting it into formats that models can interpret effectively.
Similar Read: 11 Essential Data Transformation Methods in Data Mining (2025)
Once transformed, the data may still contain unnecessary details, which makes reduction a logical next step.
Data reduction simplifies the dataset by focusing only on the most relevant information while minimizing computational load.
After these tasks, the data is ready for feature engineering in machine learning, which significantly impacts the success of your models.
Real-world data often contains irregularities, including missing values, outliers, and inconsistencies. Preprocessing transforms this chaotic raw data into a structured form that machine learning models can utilize effectively.
For instance, studies suggest that 80% of a data scientist's time is spent preparing data rather than analyzing it, highlighting its importance.
The impact of data preprocessing in machine learning extends to improving data quality, boosting model efficiency, and enabling reliable insights. Below are some critical roles preprocessing plays.
Key Benefits of Preprocessing
Preprocessing transforms raw data into a structured and reliable format, making it essential for feature engineering in machine learning. Here’s how it refines data and enhances its utility:
Similar Read: Steps in Data Preprocessing: What You Need to Know?
With a strong understanding of its importance, you can now proceed to learn the seven critical steps for effective data preprocessing in machine learning models.
Data preprocessing in machine learning is a structured sequence of steps designed to prepare raw datasets for modeling. These steps clean, transform, and format data, ensuring optimal performance for feature engineering in machine learning. Following these steps systematically enhances data quality and ensures model compatibility.
Here’s a step-by-step walkthrough of the data preprocessing workflow, using Python to illustrate key actions. For this process, we’re using the Titanic dataset from Kaggle.
Step 1: Import Necessary Libraries
Start by importing the libraries needed for handling and analyzing the dataset.
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns
Step 2: Load the Dataset
Load the dataset into a Pandas DataFrame. Make sure to download the Titanic dataset (train.csv) from Kaggle and place it in your working directory.
# Load Titanic dataset
dataset = pd.read_csv('train.csv')
# Display the first few rows of the dataset
print(dataset.head())
Output:
PassengerId Survived Pclass Name Sex Age SibSp Parch Ticket Fare Cabin Embarked
0 1 0 3 Braund, Mr. Owen Harris male 22.0 1 0 A/5 21171 7.2500 NaN S
1 2 1 1 Cumings, Mrs. John Bradley (Florence Briggs Th... female 38.0 1 0 PC 17599 71.2833 C85 C
2 3 1 3 Heikkinen, Miss. Laina female 26.0 0 0 STON/O2. 3101282 7.9250 NaN S
3 4 1 1 Futrelle, Mrs. Jacques Heath (Lily May Peel) female 35.0 1 0 113803 53.1000 C123 S
4 5 0 3 Allen, Mr. William Henry male 35.0 0 0 373450 8.0500 NaN S
Step 3: Understand the Dataset
Explore the dataset to understand its structure and identify missing or irrelevant data.
# Check the dimensions of the dataset
print(dataset.shape)
# Display summary statistics
print(dataset.describe())
# Check for missing values
print(dataset.isnull().sum())
Output:
Dimensions: (891, 12)
Summary Statistics:
PassengerId Survived Pclass Age SibSp Parch Fare
count 891.000000 891.000000 891.000000 714.000000 891.000000 891.000000 891.000000
mean 446.000000 0.383838 2.308642 29.699118 0.523008 0.381594 32.204208
std 257.353842 0.486592 0.836071 14.526497 1.102743 0.806057 49.693429
min 1.000000 0.000000 1.000000 0.420000 0.000000 0.000000 0.000000
25% 223.500000 0.000000 2.000000 20.125000 0.000000 0.000000 7.910400
50% 446.000000 0.000000 3.000000 28.000000 0.000000 0.000000 14.454200
75% 668.500000 1.000000 3.000000 38.000000 1.000000 0.000000 31.000000
max 891.000000 1.000000 3.000000 80.000000 8.000000 6.000000 512.329200
Missing Values:
PassengerId 0
Survived 0
Pclass 0
Name 0
Sex 0
Age 177
SibSp 0
Parch 0
Ticket 0
Fare 0
Cabin 687
Embarked 2
dtype: int64
Step 4: Handle Missing Data
The Titanic dataset contains missing values in columns such as Age, Cabin, and Embarked. Handle these issues appropriately:
# Impute missing 'Age' values with the median
dataset['Age'].fillna(dataset['Age'].median(), inplace=True)
# Drop the 'Cabin' column due to excessive missing values
dataset.drop(columns=['Cabin'], inplace=True)
# Fill missing 'Embarked' values with the mode
dataset['Embarked'].fillna(dataset['Embarked'].mode()[0], inplace=True)
Updated missing values:
PassengerId 0
Survived 0
Pclass 0
Name 0
Sex 0
Age 0
SibSp 0
Parch 0
Ticket 0
Fare 0
Embarked 0
dtype: int64
Step 5: Encode Categorical Variables
Convert categorical data (e.g., Sex and Embarked) into numerical formats for machine learning models.
from sklearn.preprocessing import LabelEncoder
# Encode 'Sex' column
labelencoder = LabelEncoder()
dataset['Sex'] = labelencoder.fit_transform(dataset['Sex'])
# Encode 'Embarked' column
dataset = pd.get_dummies(dataset, columns=['Embarked'], drop_first=True)
Encoded dataset preview:
Sex Age SibSp Parch Fare Embarked_Q Embarked_S
0 1 -0.558 1.0 0.0 -0.502445 0 1
1 0 0.613 1.0 0.0 0.788947 0 0
2 0 -0.257 0.0 0.0 -0.488854 0 1
3 0 0.416 1.0 0.0 0.420730 0 1
4 1 0.416 0.0 0.0 -0.486337 0 1
Step 6: Feature Scaling
Standardize numerical features such as Age and Fare to ensure uniform scaling across variables.
from sklearn.preprocessing import StandardScaler
scaler = StandardScaler()
dataset[['Age', 'Fare']] = scaler.fit_transform(dataset[['Age', 'Fare']])
Step 7: Split the Dataset
Divide the dataset into training and testing sets for model evaluation. Separate the features (X) and target (y).
from sklearn.model_selection import train_test_split
# Define features and target variable
X = dataset.drop(columns=['PassengerId', 'Name', 'Ticket', 'Survived'])
y = dataset['Survived']
# Split into training and testing sets
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)
print(f"Training set shape: {X_train.shape}")
print(f"Testing set shape: {X_test.shape}")
Output:
Training set shape: (712, 7)
Testing set shape: (179, 7)
Implementing these steps ensures your data is well-prepared for machine learning models, leading to more accurate and reliable predictions.
The first step in data preprocessing involves gathering a dataset that matches your analysis goals. The dataset should contain all the variables needed for effective feature engineering in machine learning.
Below are key points to consider when acquiring datasets:
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Popular tools like Python APIs and data repositories simplify the dataset acquisition process. Once you have the data, you can proceed to import the libraries necessary for preprocessing.
Using appropriate Python libraries ensures that data preprocessing in machine learning is both efficient and reliable. These libraries provide functions to clean, manipulate, and analyze data effectively.
Library |
Description |
NumPy | Performs numerical calculations and data manipulation. |
Pandas | Handles data frames for structured data analysis. |
Matplotlib | Visualizes data distributions and patterns. |
Example Code for Importing Libraries
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
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Each library serves a specific purpose. NumPy handles numerical data, Pandas enables data organization, and Matplotlib provides insights through visualizations. Once these libraries are imported, you can focus on loading your dataset into the environment.
Once you have acquired the dataset and imported the necessary libraries, the next step is to load the data into your workspace. This step ensures your data is ready for processing and analysis.
Below are the steps to load your dataset:
Example Code for Loading the Dataset
# Load the dataset
dataset = pd.read_csv('Dataset.csv')
# Extract independent and dependent variables
X = dataset.iloc[:, :-1].values # Features
y = dataset.iloc[:, -1].values # Target
Example output:
Step |
Output |
Load the Dataset | Displays the first few rows of the dataset. Example: pd.read_csv('Dataset.csv') shows columns like PassengerId, Survived, Age, Fare, etc. |
Independent Variables (X) | Extracted features. Example: 2D array with rows representing records and columns representing features like Age, Fare, Pclass. Example: [[22.0, 7.25], [38.0, 71.28]]. |
Dependent Variable (y) | Extracted target variable. Example: 1D array representing outcomes like [0, 1, 1, 0, 0, ...], such as the Survived column in the Titanic dataset. |
Once the data is loaded, the next step is to identify and address missing values to ensure completeness.
Detecting and addressing missing data is a critical step in data preprocessing in machine learning. Missing data, if left untreated, can lead to incorrect conclusions and flawed models. Ensuring data completeness improves the reliability of feature engineering in machine learning.
The following methods are commonly used to handle missing data.
Similar Read: Data Preprocessing In Data Mining: Steps, Missing Value Imputation, Data Standardization
Once missing data is addressed, the dataset is ready for further transformations like encoding categorical variables for machine learning models.
Machine learning algorithms require numerical inputs. Encoding categorical data into numerical form is an essential step in data preprocessing. It transforms categories into formats that algorithms can interpret effectively.
Below are the most commonly used techniques for encoding categorical variables.
Technique |
Description |
Label Encoding | Converts categories into numeric labels. Example: 'India', 'France' → 0, 1. |
Dummy Encoding (OneHot) | Converts categories into binary format with dummy variables. |
Also Read: Label Encoder vs One Hot Encoder in Machine Learning
Code Examples for Label Encoding:
from sklearn.preprocessing import LabelEncoder
labelencoder = LabelEncoder()
dataset['Sex'] = labelencoder.fit_transform(dataset['Sex'])
Before Label Encoding |
After Label Encoding |
male | 1 |
female | 0 |
male | 1 |
Code Examples for Dummy Encoding:
dataset = pd.get_dummies(dataset, columns=['Embarked'], drop_first=True)
Before Dummy Encoding |
After Dummy Encoding |
S | Embarked_Q = 0, Embarked_S = 1 |
C | Embarked_Q = 0, Embarked_S = 0 |
Q | Embarked_Q = 1, Embarked_S = 0 |
After encoding categorical variables, the dataset becomes entirely numerical and ready for outlier management.
Outliers can significantly impact model predictions and skew results. Detecting and managing them ensures that your data is consistent and reliable for analysis.
The following techniques are widely used for outlier detection and handling.
Code Example for Outlier Detection and Handling
# Z-Score Method
from scipy.stats import zscore
z_scores = zscore(dataset['Fare'])
dataset = dataset[(z_scores < 3).all(axis=1)] # Remove rows where z-score exceeds 3
Step |
Output |
Before Outlier Removal | Fare column includes extreme values such as 512.33 or 263.00, significantly higher than the mean of 32.20. |
After Outlier Removal | Rows with extreme Fare values removed; dataset is now more consistent, with Fare values within a manageable range (e.g., 0 to ~150). |
Also Read: Types of Machine Learning Algorithms with Use Cases Examples
Addressing outliers paves the way for splitting the dataset and scaling features for optimal performance.
Splitting the dataset ensures a fair evaluation of model performance. Scaling features standardizes values, ensuring each feature contributes equally during training.
Splitting the Dataset
Typical split ratios are 70:30 or 80:20. The train_test_split() function in Python simplifies this process.
from sklearn.model_selection import train_test_split
X = dataset.drop(columns=['Survived'])
y = dataset['Survived']
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)
Step |
Output |
Training Set Shape (X) | (712, n) – Contains 80% of the dataset. |
Test Set Shape (X) | (179, n) – Contains 20% of the dataset. |
Scaling Features
Feature scaling ensures uniformity in data range. This step is vital for algorithms that depend on distance metrics.
from sklearn.preprocessing import StandardScaler
scaler = StandardScaler()
X_train_scaled = scaler.fit_transform(X_train)
X_test_scaled = scaler.transform(X_test)
Step |
Output |
Standardized Features (X_train_scaled) | Values have a mean of 0 and standard deviation of 1. Example: Age is now standardized to values like -0.5, 1.2. |
Standardized Features (X_test_scaled) | Scaled using the same mean and variance as the training set to ensure consistency. |
Now the dataset is fully preprocessed and ready for training machine learning models. This sets the stage for understanding how to handle imbalanced datasets effectively.
Imbalanced datasets are common in real-world machine-learning tasks. These occur when one class significantly outweighs the other in representation, as seen in fraud detection or rare disease diagnosis.
Such disparities often lead to biased models that favor the majority class, compromising predictions for the minority class.
Below are some of the most effective methods for managing imbalanced datasets.
Aslo Read: Top Python Libraries for Machine Learning for Efficient Model Development in 2025
Imbalanced datasets pose significant challenges, but these approaches ensure fair representation and improved model performance. The next section explores how data preprocessing and feature engineering drive model performance in machine learning.
Let’s explore the critical role of data preprocessing and feature engineering in optimizing machine learning model performance.
Here are the popular techniques in feature engineering.
With these techniques, feature engineering becomes an integral part of data preprocessing in machine learning, enabling more accurate and reliable predictions.
Moving forward, the next section examines the role of data preprocessing in various machine-learning applications.
Data preprocessing in machine learning forms the backbone of AI and ML workflows. Its purpose goes beyond cleaning data to enabling efficient feature engineering in machine learning, connecting individual preprocessing steps to broader AI goals like automation, prediction accuracy, and scalability.
Below are examples illustrating how data preprocessing contributes to practical applications in machine learning and AI.
Application |
Description |
Core Elements of AI and ML Development | Clean, well-organized data enhances the development of reliable AI models. It ensures that algorithms process accurate and consistent inputs, leading to better decisions and predictions. |
Reusable Building Blocks for Innovation | Preprocessing creates reusable components such as encoded features and scaled datasets. These serve as foundational blocks for iterative improvements and new model developments. |
Streamlining Business Intelligence Insights | Preprocessed data provides actionable insights by identifying patterns and trends. It helps businesses optimize operations, forecast outcomes, and make data-driven decisions. |
Improving CRM through Web Mining | Web mining relies on preprocessed data to analyze customer interactions online. This enhances customer relationship management by identifying preferences and improving user experience. |
Personalized Insights through Session Tracking | Preprocessing user session data enables better understanding of customer behavior. It identifies preferences, session durations, and interactions, contributing to tailored recommendations and personalized marketing. |
Driving Accuracy in Consumer Research | Accurate preprocessing provides researchers with high-quality datasets, enabling deep insights into consumer behavior and preferences. This leads to better product designs and targeted marketing campaigns. |
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The next section will discuss key professionals in feature engineering and data preprocessing and explore their salary trends.
Professionals specializing in data preprocessing in machine learning and feature engineering play pivotal roles in preparing and optimizing data for advanced analytics. Their expertise ensures that raw data is transformed into actionable inputs for machine learning models, directly impacting innovation and decision-making.
Below are the primary roles involved in this domain, along with their responsibilities and average annual salaries.
Role |
Description |
Annual Average Salary (INR) |
Data Scientists | Develop predictive models, analyze data patterns, and lead machine learning projects. | 12L |
Data Engineers | Design and maintain infrastructure for data collection, storage, and transformation. | 8L |
Machine Learning Engineers | Build and deploy machine learning models, focusing on algorithm optimization and scalability. | 10L |
Business Analysts | Interpret data trends to inform business strategies and decisions. | 8L |
Data Analysts | Clean, manipulate, and visualize data to support analysis and reporting tasks. | 6L |
Data Preprocessing Specialists | Specialize in preparing datasets by handling missing data, scaling, and encoding features. | 3L |
Data Managers | Oversee data governance, quality, and compliance within organizations. | 10L |
Source: Glassdoor
Next, let’s explore top strategies for effective data preprocessing and feature engineering in machine learning.
Effective data preprocessing and feature engineering in machine learning are essential for transforming raw data into valuable insights. These strategies streamline the preparation process, enhance model accuracy, and ensure robust predictive capabilities across diverse applications.
The next section delves into transformative applications of data processing in machine learning and its role in driving business outcomes.
Data preprocessing in machine learning and feature engineering in machine learning are driving innovations across industries. They enable businesses to derive actionable insights, enhance decision-making, and deliver personalized customer experiences.
The applications span various domains, showcasing how effective data handling impacts both operational efficiency and strategic growth.
Application |
Description |
Customer Segmentation | Data preprocessing helps group customers based on behaviors and preferences, enabling targeted marketing strategies. |
Predictive Maintenance | Feature engineering identifies patterns in sensor data to predict equipment failures, reducing downtime. |
Fraud Detection | Machine learning models trained on preprocessed transaction data detect anomalies and fraudulent activities effectively. |
Healthcare Diagnosis | Clean and structured medical data supports accurate disease detection and personalized treatment recommendations. |
Supply Chain Optimization | Feature engineering optimizes logistics by forecasting demand and minimizing inefficiencies. |
Recommendation Systems | Preprocessed user data improves recommendation engines for e-commerce and streaming platforms, boosting engagement. |
Sentiment Analysis | Text preprocessing enables sentiment classification, helping brands analyze public perception and customer feedback. |
Next, the focus shifts to how upGrad can enhance your data processing and machine learning expertise to advance your career and technical skills.
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We empower you to excel in cutting-edge domains like data preprocessing in machine learning and feature engineering in machine learning. Below are some of the courses you can explore to strengthen your expertise in these fields.
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Dataset:
Titanic: https://www.kaggle.com/c/titanic/data
Reference:
https://www.dataversity.net/survey-shows-data-scientists-spend-time-cleaning-data
https://www.glassdoor.co.in/Salaries/data-scientist-salary-SRCH_KO0,14.htm
https://www.glassdoor.co.in/Salaries/data-engineer-salary-SRCH_KO0,13.htm
https://www.glassdoor.co.in/Salaries/machine-learning-engineer-salary-SRCH_KO0,25.htm
https://www.glassdoor.co.in/Salaries/business-analyst-salary-SRCH_KO0,16.htm
https://www.glassdoor.co.in/Salaries/data-analyst-salary-SRCH_KO0,12.htm
https://www.glassdoor.co.in/Salaries/data-processing-specialist-salary-SRCH_KO0,26.htm
https://www.glassdoor.co.in/Salaries/data-manager-salary-SRCH_KO0,12.htm
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