Data Leakage In Machine Learning: Examples & How to Protect

May 10, 2024
20 Mins

Machine learning algorithms often show remarkable success in training, achieving impressive levels of accuracy and performance. However, when these algorithms are deployed in real-time production environments, they often encounter difficulties and can provide unreliable and inaccurate predictions. Data leakage is a crucial element contributing to this discrepancy, leading to biased or overly optimistic results.

This article helps you understand what causes data leakage in machine learning and offers best practices to mitigate it.

What is Data Leakage in Machine Learning?

Data leakage in machine learning occurs when information from outside the training dataset is unintentionally utilized during the model creation process. This leakage can have detrimental effects on the model's predictions and its ability to generalize unseen data, resulting in unreliable and inaccurate predictions.

Data Leakage

Data leakage can lead to overly optimistic results as the model may learn patterns or relationships that are not representative of real-world scenarios. This compromises the reliability and accuracy of the model's performance, highlighting the importance of identifying and mitigating data leakage to ensure robust machine learning models.

What Causes Data Leakage?

Data leakage in machine learning can occur due to various factors. Here are some common causes of data leakage:

Inclusion of Future Information: When the model includes information that would not be available at the time of prediction in a real-world scenario, such as using future data to predict the past, this can lead to data leakage.

Inappropriate Feature Selection: Selecting features highly correlated with the target variable but not causally related can introduce data leakage. Including such features can allow the model to exploit this correlation and make predictions based on information it should not have access to in real-world scenarios.

External Data Contamination: If external datasets are merged with the training data, ensuring that the added information does not introduce data leakage is crucial. External data can sometimes contain direct or indirect information about the target variable, leading to biased or inaccurate predictions.

Data Preprocessing Errors: These can occur when scaling the data before dividing it into training and validation sets or when imputing missing values using information from the entire dataset. This can expose information about the validation or test data to the model during training, leading to data leakage.

Impact of Data Leakage on Machine Learning Models

Data leakage can significantly impact machine learning models, affecting their performance, reliability, and generalization capabilities. Here are some key impacts of data leakage:

Poor Generalization to New Data

Models affected by data leakage often struggle to generalize well to unseen data. Since the leaked information does not represent the real-world distribution, the model's predictions on new, unseen data may be inaccurate and unreliable. This compromises the model's ability to make meaningful predictions in practical applications.

Biased Decision Making

Data leakage can introduce biases into the model's decision-making process. If the leaked information contains biases or reflects specific circumstances that do not apply universally, the model may exhibit skewed behavior, making decisions that are not fair or aligned with real-world scenarios.

Unreliable Insights and Findings

Data leakage can compromise the reliability and validity of insights and findings derived from the machine learning model. When leakage occurs, the relationships and correlations discovered by the model may not be reflective of the true underlying patterns in the data. This can undermine the trust and confidence in the model's output, making it difficult to rely on its predictions.

Data Leakage Examples

Here are some examples of data leakage in machine learning:

Overfitting Due to Target Leakage: This occurs when a model is being trained to predict a target variable, but the training data includes information about the target variable that the model would not have access to during deployment. For example, if you're training a model to predict whether a customer will churn. However, your training data accidentally includes whether the customer canceled the subscription. The model may memorize the training data and will perform poorly on new data as it has yet to truly learn the patterns that lead to cancellations.

Optimistic Performance Due to Train-Test Data Leakage: Suppose you're building an image classification model to distinguish between cats and dogs. If some of the images in your test set also appear in your training data, the model may perform well during testing because it has seen similar images before. However, this doesn't reflect its actual performance on completely new and unseen images.

Biased Predictions Due to Data Preprocessing Leakage: For example, if you're training a model to predict loan approvals. During data preprocessing, you mistakenly scale the loan amounts using the maximum value from the entire dataset, including the test set. This introduces leakage because the model has access to information (the maximum value) that it shouldn't have during deployment. As a result, the model might give more weight to larger loan amounts, leading to biased predictions when faced with new data.

How to Prevent Data Leakage in Machine Learning?

Here are some best practices that can significantly reduce the risk of data leakage and help you build more reliable and robust machine learning models:

Proper Data Splitting: It is crucial to separate your data into distinct training and validation sets. Doing so ensures that no information from the validation set leaks into the training set or vice versa. This separation ensures that the model is trained only on the training set, allowing it to learn patterns and relationships in the data without any knowledge of the validation set. 

Cross-Validation: Proper cross-validation helps mitigate data leakage and ensures reliable model evaluation. One commonly used approach is k-fold. The dataset is partitioned into k folds, where each fold serves as the validation set once, while the remaining k-1 folds are used as the training set. This ensures that the model is consistently evaluated on different data subsets across multiple iterations. 

Feature Engineering: Feature engineering should be carried out exclusively using the training data. It is crucial to prevent utilizing any information from the validation or test sets to create new features, as this can lead to data leakage.

Data Preprocessing: Avoid preprocessing the data based on the entire dataset. Scaling, normalization, imputation, or any other data preprocessing steps should be performed solely on the training set.

Time-based Validation: The dataset should be split into training and validation sets based on the chronological order of the data points. It helps prevent data leakage by ensuring that the model only learns from past information. This prevents the use of future information to predict past events, which could lead to overly optimistic performance estimates.

Regular Model Evaluation: Continuously monitor and evaluate the performance of your model on new, unseen data. This helps identify any potential leakage issues or performance degradation over time. 

Streamline Your Machine Learning Workflow with Airbyte

To avoid data leakages, machine learning workflows rely on efficient data pipelines to process vast amounts of information. Pipelines provide a structured and automated approach to collecting and processing data, ensuring that sensitive information is handled securely. One such platform that simplifies the process of building data pipelines is Airbyte. It offers a user-friendly interface where you can easily configure and manage data integration workflows without extensive coding knowledge.


Let’s explore the key features of Airbyte:

Custom Connectors: Airbyte offers a vast library of over 350+ pre-built connectors that allow you to seamlessly integrate various data sources, ensuring efficient and secure data transfer without the risk of leakage. Furthermore, if you don’t find the desired connector, Airbyte empowers you with even greater flexibility through its Connector Development Kit (CDK). With the CDK, you can quickly build custom connectors in less than 30 minutes.

Transformations: Airbyte adopts the ELT (Extract, Load, Transform) approach, which involves loading data into the target system prior to transformation. However, it allows you to seamlessly integrate with dbt (data build tool), empowering you to perform advanced and customized data transformations.

PyAirbyte: Airbyte introduced PyAirbyte, a Python library that allows you to interact with Airbyte connectors through Python code.

Data Security: Airbyte prioritizes the security and protection of your data by adhering to industry-standard practices. It employs encryption methods to safeguard data in transit and at rest. Additionally, it incorporates robust access controls and authentication mechanisms, guaranteeing that only authorized users can access and utilize the data.

Wrapping Up

Mitigating data leakage is of utmost importance in machine learning to maintain model accuracy and performance. To overcome this, it is imperative to implement best practices such as feature engineering, meticulous data splitting, and leveraging robust data pipelines. Data pipelines play a crucial role in maintaining the integrity and consistency of data, facilitating the detection and prevention of data leakages. 

Consider using a data integration platform like Airbyte to streamline and optimize your workflows. Sign up today to explore its powerful features.

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