Data Preprocessing: How To Process Your Data For Optimal Performance

Data preprocessing is one of the early steps of creating and utilizing a machine learning model. In this step, the raw data is prepared to be suitable for feeding to the machine learning model. It is often the first step undertaken when creating a machine learning project, as the availability of clean and well-formatted data is not always possible.

Table of Contents

• Data Preprocessing Features

• Uses and Importance of Data Preprocessing in Machine Learning

• Data Preprocessing Steps/Stages

• Best Practices for optimized Data Preprocessing in Machine Learning

The data preprocessing process consists of any action to make the input data compatible with the machine learning model. These actions can include data cleaning, formatting, data reduction, finding missing data, data enhancement, and more.

This process is also one of the initial steps in other data analysis tasks, such as data mining and data analysis, as analytical applications require formatted data that can be understood by the computers and the machine learning model used.

The raw input data that goes into the data preprocessing process can be any data, such as text, images, video, and so on. It can be unstructured, structured, or a combination of unstructured and structured data. Much of this data comes from various sources that can be gained via data mining and warehousing techniques. Any raw data is transformed into the format and order the machine learning model requires for optimized data analysis.

Data Preprocessing Features

Machine learning models operate on datasets with the help of data properties or features. A feature is an independent variable with a certain value representing a particular dataset attribute. For instance, in the case of a dataset containing personnel details, the person’s name, age, sex, role, and qualifications can all be considered features. Each machine learning model is trained to work with certain features and derive its predictions and insights based on these features. Data preprocessing in machine learning helps narrow down or clean out the raw data into focused datasets with the necessary features that can be easily operated upon by a machine learning model.

Features can be broadly classified into two types:

• Categorical features

Features whose values are derived from a fixed, defined set of possible values or explanations are called categorical. They can have any definitive or descriptive value, such as date, Boolean (true or false), positive, neutral, and types.

• Numerical features

These features contain values that can be numerically associated on a continuous scale or statistically related. Any number, fractional value, or percentage, such as income, the number of words in a document, time duration, and so on, can be classified as a numerical feature

Tip:

While data preprocessing is a critical step in the machine learning process, it’s important to remember that not all data sets are created equal. In order to get the most out of your machine learning model, be sure to use high-quality datasets that have been pre-processed for optimal performance.

More about Datasets for Machine Learning

Uses and Importance of Data Preprocessing in Machine Learning

Data preprocessing in data mining is a crucial step in creating and training machine learning models. It is essential to ensure that the machine learning model works with valid data and can thus provide accurate results and predictions.

• Removed noise

Most real-world data come with inherent noise and various kinds of formats and might be incomplete. They are collected from various sources and combined to form a huge data set with many inaccuracies, inconsistencies, and raw data. Feeding them directly into a mathematical model is nearly impossible. Data preprocessing takes care of filtering out the data, formatting it, and cleaning it so that only valid and suitable data is used in the machine learning models.

• Easy data consumption

Even when the input data is structured, it may still not have the same fields and properties required for a particular problem that the machine learning model tries to solve. Data preprocessing in machine learning helps prepare data in the right way so that it can be readily consumed for further analysis.

• Improves accuracy

Machine learning models run on data and completely rely on the data they use to remain accurate and unbiased. The more data you have, the better you can train your machine-learning model. Without such data preprocessing steps, we will be unable to ensure the accuracy and legitimacy of the results we gain from the machine learning model. It also considers outliers and inconsistent data points, reducing false predictions.

• Improves performance

Data preprocessing allows for better accuracy and eliminates several bottlenecks in data analysis by making the input data sets more relevant and easier to parse. It helps improve the machine learning model’s performance by providing clean data that can be processed faster.

The quality of a machine learning model is evaluated based on the quality of its results. High quality cannot be achieved without the help of proper data preprocessing in machine learning. If you use dirty data to train your model, you will end up with a model that produces no useful results. Hence, data preprocessing is considered a crucial and mandatory step in machine learning.

Data Preprocessing Steps/Stages

The basic data preprocessing steps in machine learning are:

Data Cleaning

Data cleaning involves basic operations such as filling in the missing values, removing noise, and removing inconsistencies and outliers from the input data. There are many techniques used for each of these operations.

Missing values could be resolved by either ignoring the tuples with missing values or filling them with proper values either manually or through a predictive model.

Noise in data can be handled by using binning, regression, and clustering techniques.

Outliers can be removed by clustering the data into groups.

Data Integration

As mentioned earlier, input data can be aggregated from multiple sources. But doing so would require you to handle the inconsistencies in format and missing values that could arise from combining the various datasets. The data integration part of data preprocessing takes care of this by merging the data from multiple sources into a single data store. This process is similar to how a data warehouse operates.

Data collected from different sources must be integrated into a single large database and then worked upon to smooth out the noise and inconsistencies. Some usual problems you might face when trying to merge datasets could be:

• Schema integration and object matching: Variations in formats and data attributes could make it difficult to merge data into a single database.
• Redundancy: Duplicate and redundant data should be removed from all sources.
• Data value conflicts: Different sources could give conflicting data values for the same attribute, and the correct value must be determined.

Data Transformation

Data consolidated from multiple sources will have to be transformed into a more acceptable format with the help of transformation strategies.

• Generalization

The collected low-level data are transformed into high-level information with the help of concept hierarchies. For instance, address data collected from customer information can be organized into country-level hierarchies.

• Normalization

There are multiple methods to normalize data, such as mi-max normalization, z-score normalization, and decimal scaling normalization. In normalization, the numerical attributes of data are normalized to fit within a particular range of values. Several data points can also be transformed into a single data attribute that fits into an acceptable range of values. Thus, the inconsistencies and differences between various data values are resolved.

For example, when huge numerical values are presented for different attributes, the values can be made to fall under a range of 0 to 1 by applying a common denominator. Take the example of a data set with two features: age and income. Age usually ranges from 0 to 100 values, whereas income values go higher than 6-digit values. These two data features can be normalized in the same range of 0 to 1 using min-max scalar normalization.

• Attribute selection

A data set may contain a lot of attributes that the machine learning model does not necessarily consider. There could also be new properties added to the combined dataset. Attribute selection is performed to retain only the required features.

• Aggregation

Aggregation is performed to get a summary of the datasets by correlating one or more features. For instance, a sales dataset can be summarized to show sales data per month or year.

Data Reduction

While it is true that more data means more accuracy, the quality of the data is what counts. Just a huge quantity of redundant data will not help increase the accuracy of the learning models. And having a lot of data to process can also slow down the machine-learning model’s performance. One good way to achieve high-quality results without sacrificing performance is to perform data reduction or sampling during the data preprocessing stage. Data reduction helps get a reduced quantity of data that produces the same quality of results. Some of the techniques used are

• Data cube aggregation

Data is presented in a summarized format.

• Dimensionality reduction

This technique allows for extracting only the required feature and eliminating redundant features. Techniques such as principal component analysis help reduce the number of features and only retain the necessary ones. Too many features or too few features can cause problems like overfitting or underfitting while training the machine learning models.

• Data compression

Data compression helps efficiently store the huge machine learning datasets. These techniques use encoding technologies and can be lossy or non-lossy. If the original data is retained after compression, it is called non-lossy/lossless compression. If any data is lost during the data compression process, it is called “lossy compression.”

• Discretization

Data discretization is similar to summarizing data, where data of a continuous nature is divided into groups of particular ranges. For instance, personnel data can be grouped in terms of income brackets.

• Numerosity reduction

If data can be simplified and represented as an equation or a mathematical model, it is called numerosity reduction. This method is hugely helpful in reducing the storage space required.

• Attribute subset selection

Besides selecting the particular attributes, further optimization can also be achieved by selecting the specific subset attributes of each attribute.

Data Quality Assessment

A quality assessment of the data is performed to ensure the input data does not contain any issues. This includes checking for the validity and consistency of data across all its features. As the insights derived from machine learning are used in real-world decision-making, it is of utmost importance that the input data is of high quality. The three main activities involved in data quality assurance are

• Data profiling: Investigating the dataset for any quality issues
• Data cleaning: Fixing the found data issues

Data monitoring: Ensuring that data is maintained in a clean state and continuously checking whether the available data meets its intended needs.

Best Practices for optimized Data Preprocessing in Machine Learning

• Get a good understanding of the concept

Before getting into data preprocessing in machine learning, it is important to understand the purpose of the machine learning model under consideration. You need to have a good idea of the exact business needs and expectations you seek to satisfy and correlate them to the data to be collected and processed.

• Make use of statistics and pre-built libraries

Standardized data preprocessing methods such as statistical models and pre-built libraries allow you to save time and have assured results.

• Summarization

Summarizing data in terms of duplicates, missed values, outliers, and so on can give you a good idea of how much effort it takes to pre-process the data. You can thus go ahead with the preprocessing with a good estimate of the resources required.

• Dimensionality reduction to feature engineering

Understanding the problem you intend to solve will help you identify the necessary attributes to design the machine learning model. Using too many unnecessary attributes will slow your models and affect their quality. Make sure to cut down on the attributes used and clarify what is required to make your data preprocessing efficient and faster. Feature engineering helps you achieve this by helping you identify the attributes that are most useful for your machine-learning project.

Data preprocessing thus plays an important role in machine learning, cleaning the raw data and making it suitable for machine learning processing.

Robert Koch