Mastering data preprocessing: Techniques and best practices
Data preprocessing is a critical step in the data science process, and it often determines the success or failure of a project. Preprocessing involves transforming messy, unstructured, and noisy data into a structured format suitable for computers to read and analyze. It is essential to ensure that the data is ready for predictive modeling or other machine learning tasks.
This article will explore different types of data preprocessing techniques and best practices for mastering them. We’ll also look at how to use Python to perform these tasks more effectively. By this article’s end, you will better understand why data preprocessing is essential and how to master it. Data Preprocessing for Machine Learning
Data preprocessing is an essential step that serves as the foundation for machine learning. It involves taking raw data and transforming it into a usable format for analysis and modeling.
Data is cleaned, structured, and optimized through data preprocessing to ensure optimal performance when used in machine learning algorithms. By preparing the data correctly, we can maximize the accuracy of our predictions or classifications.
Overview of Preprocessing Techniques
We can divide data preprocessing techniques into several steps, including data cleaning, data transformation, and data integration.
Data cleaning involves removing missing values and duplicates, while data transformation involves scaling and normalizing the data, encoding categorical variables, and handling outliers. Finally, data integration consists of merging datasets and taking imbalanced data.
The fundamental concepts of data preprocessing include the following:

Data cleaning and preparation

Categorical data processing

Variable transformation and discretization

Feature extraction and engineering

Data integration and preparation for modeling.
We will take a look at each of these in more detail below.
Data Cleaning and Preparation
Data cleaning and preparation is the first step in data preprocessing. It involves identifying missing values, incorrect values, outliers, and other inconsistencies that can affect the accuracy of machine learning algorithms.
Let’s take a closer look at individual tasks and how to approach them when preprocessing your datasets.
Dealing with Missing Values
Missing values are a common problem in datasets. These occur when data is unavailable, or there is a lack of information in the dataset. Identifying and dealing with missing values is essential as they can lead to inaccurate results when using machine learning algorithms.
There are several techniques for dealing with missing values, including dropping the rows or columns containing them, imputing the values using a simple method such as mean/median/mode, filling in the missing value based on other records that have similar data points, or predicting them using supervised machine learning methods.
Missing Data Imputation Techniques
Imputation is a statistical process of replacing missing data with substituted values. The most commonly used methods for imputation are mean/median/mode substitution, knearest neighbor (KNN) imputation, and multiple imputations by chained equations (MICE).
Mean/median/mode substitution involves filling each missing value with the mean or median of all other nonmissing values in that column. On the other hand, KNN imputation uses the knearest neighbors algorithm to predict missing values based on their similarity to different values in the dataset. Lastly, MICE is a more advanced method that uses regression models and multiple imputations to fill in missing values.
Removing Duplicates and Inconsistent Data
Duplicates can lead to the overrepresentation of data, which can negatively impact the performance of machinelearning models. Inconsistent data, such as recorded in different units, can also affect the accuracy of machine learning models. Data cleaning techniques, such as standardization, can help to address these issues.
Categorical Data Processing
Categorical data is a type of data that has been grouped into categories. It can be either nominal (no order) or ordinal (with order). Examples of categorical data include gender, race, marital status, and job titles.
Categorical Encoding Techniques
Categorical encoding is the process of transforming categorical data into numerical values. This is often done using onehot encoding, which creates a binary vector for each category and assigns a value of 1 or 0, depending on whether that category is present.
You can also use other techniques like label encoding by assigning numeric values to categories to reduce the dimensionality of the dataset.
Binary encoding is another technique that binary code, that is, a sequence of zeroes and ones, to represent the different categories of the variable.
The choice of encoding technique depends on the nature of the categorical data and the goal of the analysis.
Handling Ordinal Categorical Variables
Ordinal categorical variables are categorical variables that have an order or hierarchy, such as high, medium, and low. Examples of ordinal data include education levels (e.g., high school, college, graduate), customer satisfaction ratings (e.g., 15 stars), or letter grades (A+, A, B, C).
When dealing with ordinal categorical variables, it is often necessary to define the relative importance of each category before encoding them as numeric values. You can assign a numerical value to each level and then normalize them so they range between 0 and 1.
Dealing with High Cardinality Categorical Variables
High cardinality categorical variables are categorical variables with many unique categories, such as street names or product names. These variables can be challenging to process as they can result in highdimensional data, which can negatively impact the performance of machine learning models.
You can employ techniques such as dimensionality reduction and feature selection to address such difficulties. Alternatively, you can encode only the most frequent categories from such variables.
Variable Transformation and Discretization
Variable transformation consists in changing the distribution of a continuous variable by applying a mathematical transformation like the logarithm, or the squareroot. This is usually done to satisfy the assumptions made by some statistical model.
Discretization is a technique that divides a continuous variable into discrete categories or bins. Discretization transforms a continuous variable into a categorical one (for example, turning age into a range). This data preprocessing step can help simplify our model by reducing the number of values we have to work with (useful to train decision trees faster).
Scaling and normalization consist in changing the value range of the variable, and it is usually a requirement for some machine learning models.
There are several variable transformation and discretization techniques we can follow in our data preprocessing, as follows:
Scaling and Normalization
Feature scaling or normalization is the process of changing the range or scale of our data. This can help ensure all variables are on the same scale and allow us to compare them more easily. It is also a requirement for some machine learning models.
Some normalization methods transforms a variable’s values into a range between 0 and 1. Other methods help ensure that outliers don’t excessively influence our models’ performance.
We can use various scaling and normalization techniques, such as minmax scaling, mean normalization, and unit vector transformations.
Binning and Discretization
Let’s find out how binning and discretization work with a data preparation example.
Consider realworld data of the ages of 1000 people, with the ages ranging from 18 to 90. Using binning, data scientists can group the ages of the original data into smaller categories, such as 1830, 3145, 4660, and 6190. This would improve data quality through a transformed variable with four categories instead of 1000 numerical data.
Using discretization, we could transform the ages into a categorical variable with three c ategories: young, middleaged, and old. To do this, we would first choose the cutoff points between the categories, such as 30 for young, 45 for middleaged, and 60 for old. We would t hen assign each person to a class or subset based on age, resulting in a transformed variable with three categories.
This example illustrates how binning and discretization can simplify continuous variables and make them easier to work with in machine learning models and data analysis.
Outlier Detection and Handling
Outliers are data points that lie far away from a dataset’s main cluster of values. Errors or extreme values in your dataset can cause outliers. Identifying and handling them is crucial as they can hurt our machine learning models.
We can detect outliers using statistical methods such as zscores or box plots, and we can then either remove them from the dataset or transform them into more reasonable values.
Several techniques for detecting and handling outliers include removal, imputation, and capping.

Removal: While removing outliers, we must ensure that the data points being removed are indeed outliers, not just extreme values.

Imputation: Instead of removing the outliers, we replace them with more reasonable values. We can do it by imputing those data points with the variable median or mean values.

Capping: In this case, we set a maximum and minimum threshold, after which any data point will no longer be considered an outlier. All values outside these thresholds are then replaced by the threshold value (either max or min).
Feature Extraction and Engineering
Feature extraction and engineering involve transforming and creating new features from existing data collection. This can include combining different columns, aggregating data from transactions or time series into meaningful features, or extracting meaningful information from text documents.
Creating New Features from Existing Ones
Feature engineering aims to create new features that are more useful for predictive modeling, big data analysis, and artificial intelligence than the original ones. By constructing better features, we can increase the accuracy of our models and make them more robust to changes in the data values.
For example, creating a new feature that represents the total number of years of education and experience a person has could provide more meaningful information than just the years of education or experience individually.
The Feature Engineering course for Machine Learning comprehensively covers many details of discretization techniques, outlier handling, data imputation methods, and more!
Dimensionality Reduction Techniques
Dimensionality reduction techniques help reduce the complexity of data sets by combining features into a single or fewer variables. This can reduce the size of a data set, improve model accuracy, and reduce computational costs.
Standard dimensionality reduction techniques include:

Principal component analysis (PCA)

Singular value decomposition (SVD), and

Linear discriminant analysis (LDA).
Feature Selection and Importance Evaluation
Feature selection involves selecting a subset of the essential features, while feature importance evaluation consists of evaluating each component’s importance and ranking them. This important step can help improve the performance of machine learning models and reduce the risk of overfitting and redundancies of training data.
The Feature Selection course for Machine Learning comprehensively covers many feature selection methods!
Data Integration and Preparation for Modeling
Data integration and preparation for modeling is the final step of data preprocessing. It involves combining different pieces of data, such as text or numerical values, into one unified dataset suitable for machine learning algorithms. We can then split the data into training sets to prepare it for predictive modeling.
Steps of data integration include:

Merging datasets: By combining different data sources, we can uncover valuable insights that would otherwise be hidden. For example, by merging customer purchase history and demographic information, we can gain an understanding of our customers’ buying behaviors.

Dealing with large datasets: Large datasets often require special processing techniques to ensure the data is accurate and efficient. For example, sampling can be used to reduce the size of a dataset without compromising accuracy. Other methods, such as feature selection, dimensionality reduction, and numerosity reduction, can also help manage large datasets for neural networks.
Once the data has been integrated and prepared, we can use it in a machinelearning algorithm. The algorithm’s accuracy depends on how effectively the data has been preprocessed; errors with data mining techniques during preprocessing could significantly decrease model performance.
Preprocessing Techniques in Python
Python is a popular programming language for data science, and it has many powerful libraries that make preprocessing tasks easier.
Python libraries such as these make it easy to master data preprocessing quickly. Here are some quick tips and tricks for Effective Data Preprocessing in Python:

Know your data: Before preprocessing your data, it is essential to understand the data structure, the types of variables, and the distribution of the data.

Use the correct libraries: Choose the right libraries for the preprocessing techniques you need to use. For example, use pandas for data manipulation, NumPy for numerical computations, and scikitlearn for machine learning algorithms.

Automate preprocessing: Use functions, scripts, and pipelines to automate preprocessing tasks. Outsource feature engineering to python libraries like tsfresh, Featureengine, Category encoders and Featuretools.

Test and validate preprocessing: Test and validate your preprocessing steps to ensure that the data is preprocessed correctly and that the results are accurate.
Final Thoughts
Data preprocessing is a fundamental step in the data science process, and it can make or break a machine learning project. Understanding the different preprocessing techniques and best practices for mastering them is essential.
From dealing with missing values, transforming variables, and extracting features to integrating datasets and automating the process with Python, it is important to consider each step in the preprocessing phase carefully.
Check out the Python Feature Engineering Cookbook for over 70 detailed stepbystep tutorials on building machine learning models.