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Validating Machine Learning Models: A Detailed Overview

MarkovML
January 10, 2024
11
min read

The industry is evolving, and every business area is using machine learning or artificial intelligence in some form. But while ML models are becoming increasingly sophisticated and complex, their accuracy is still subjective. In most implementations, an accuracy level greater than 70% is considered great, while an accuracy measure of 70%-90% is considered ideal as per industry standards.

This leaves a significant risk of errors or outputs going wrong, which may not be what is expected in the real world. As a result, it is more important than ever to validate their performance before deploying them in production.

ML model validation evaluates a model's performance on data that was not used to train the model. This helps to ensure that the model will generalize well to new data and perform as expected in the real world.

Let us understand this in greater detail.

Why Validate Machine Learning Models?

There are several reasons why it is important to validate ML models.

First, ML model validation can help to identify and correct overfitting. Overfitting occurs when a model learns the training data too well and is unable to generalize to new data. Model validation can help to identify overfitting by evaluating the model's performance on data that was not used to train it.

Second, ML model validation is used to select the best model for a given task. There are many different types of machine learning models, each with its own strengths and weaknesses. Model validation can help compare the performance of different models on a given dataset and select the model most likely to generalize well to new data.

Third, ML model validation also helps to track the performance of a model over time. As the data distribution changes, the performance of a model may also change. Model validation can help to identify these changes and take corrective action if necessary.

Types of Model Validation

Evaluating a model's performance on unseen data is crucial for ensuring its generalizability and real-world effectiveness. Several techniques exist, each with its advantages and limitations. Let's explore some common methods:

Train-Test Split

Train test split is an ML model validation method where you can simulate how the model behaves when it is tested using new or untested data. Here is an example of how the procedure works:

Train Test Split: What it Means and How to Use It | Built In
Source

This approach divides the data into two sets: training (used to build the model) and testing (used for evaluation). While straightforward, it can lead to unstable estimates if the split is not representative of the overall data distribution.

K-Fold Cross-Validation

K-Fold cross-validation is a process that works on the train-test split model, where the data is divided into ‘k’ equal parts, as shown in the image below.

Understanding Cross Validation's purpose | by Matthew Terribile | Medium
Source

Similar to train-test split, in K-Fold, your dataset is partitioned into 'K' equally sized folds. The ML model then trains on 'K-1' folds and validates on the remaining one. This process repeats 'K' times, with each fold taking a turn as the validation set. It ensures thorough learning across the entire dataset.

Stratified K-Fold Cross-Validation

Stratified K-fold cross-validation ensures each fold contains a representative proportion of each class for datasets with imbalanced classes, where one class dominates the others. It shuffles your data and then splits it into ’n’ different parts. Now, it will use each part to test the model and only shuffle data one time before splitting.

A bar graph demonstrating a stratified K-fold cross-validation
Source

This prevents the model from favoring the majority class and provides a more accurate assessment of its performance across all classes.

Leave-One-Out Cross-Validation (LOOCV)

A version of the K-fold Cross Validation model, LOOCV is a popular technique where the entire dataset is partitioned into folds. Each data point becomes its own test set, and the model is trained on the remaining data.

Leave-One-Out Cross-Validation. Extreme version of k-fold… | by Naina  Chaturvedi | DataDrivenInvestor
Source

While LOOCV provides the most accurate performance estimate, it can be computationally expensive for large datasets.

Holdout Validation

Similar to train-test split, holdout validation involves setting aside a portion of the data for evaluation. However, this portion is held out during the entire training process and only evaluated once the final model is built.

A split of a dataset into training and testing sets with corresponding actions
Source

This can be useful for datasets that are constantly updated, as the holdout set can be used to evaluate the model's performance on the most recent data.

Time Series Cross-Validation

Cross-validation in time series is a procedure designed specifically for time series data. This technique utilizes overlapping windows. The model trains on one window and evaluates on the next, moving sequentially through the data. This accounts for the inherent temporal dependencies present in time series data and provides a more accurate assessment of the model's ability to predict future values.

Cross Validation in Time Series. Cross Validation: | by Soumya Shrivastava  | Medium
Source

It mimics real-world scenarios where past data is used to predict the future, preventing the model from peering into the future during training.

Metrics for Model Validation

We have seen multiple ways to test and validate ML models. However, once you have chosen a validation technique, the success of this model hinges on selecting the right metrics to track its performance.

These metrics can be broadly categorized into two main groups:

Error-based Metrics

  • Mean Squared Error (MSE): Measures the average squared difference between predicted and actual values.
  • Mean Absolute Error (MAE): Measures the average absolute difference between predicted and actual values.
  • Root Mean Squared Error (RMSE): The square root of MSE helps interpret errors in the same units as the target variable.

Classification-specific Metrics

  • Accuracy: Proportion of correct predictions.
  • Precision: Proportion of positive predictions that are actually positive.
  • Recall: Proportion of actual positives that are correctly identified.
  • F1-Score: Harmonic mean of precision and recall, balancing both metrics.

To demonstrate how we can use metrics for model validation, take the example of a validation focusing on classification as the analysis objective. In this case, you can use the F1-score to understand how well the model is doing in this regard. Meanwhile, if you are doing regressions, you may use R-squared or mean-squared error for the analysis.

Handling Imbalanced Datasets during Validation

While traditional metrics like accuracy, precision, and recall are widely used in model validation, they can be misleading when dealing with imbalanced datasets. This tendency to favor particular datasets leads to imbalanced datasets, often leading to an inaccurate picture of the model’s performance.

Some of the reasons why this happens can be:

1. Bias towards the Majority Class

Traditional metrics often favor the majority class. This can lead to overlooking essential errors and potentially impacting decision-making based on skewed results.

For example, a model with 99% accuracy on a dataset with 99% negative examples and 1% positive examples might seem highly accurate. However, it could misclassify all positive examples, leading to misleading conclusions about the model's effectiveness in identifying the minority class.

2. Masking the Actual Performance of the Minority Class

Another issue that can occur is when the model masks the actual performance of the minority class. This can be problematic when identifying rare events or anomalies, where accurate classification of the minority class is crucial.

For instance, a fraud detection model must have high accuracy and be able to detect even granular deviations or anomalies in the system. Sometimes, masking of majority transactions may lead to subtle fraudulent activities getting missed if the majority of transactions are legitimate. Relying solely on accuracy might mask this issue, leading to a false sense of security.

Solutions and Alternatives

To address these challenges, it's crucial to use metrics that are designed explicitly for imbalanced datasets. These include:

  • F1-Score: Provides a harmonic mean of precision and recall, accounting for both metrics and balancing their importance.
  • G-Mean: Computes the geometric mean of sensitivities for each class, providing a better overall picture of performance across all classes.
  • AUC-ROC: Measures the model's ability to discriminate between classes, offering a robust evaluation independent of class distribution.
  • Precision-Recall Curves: Visualize the trade-off between precision and recall across different thresholds, enabling a deeper understanding of the model's performance under various scenarios.

Model Interpretability and Explainability in Validation

Achieving accurate predictions isn't the sole goal of responsible AI development. Understanding how and why an ML model makes certain decisions is equally important. This is where interpretability and explainability come into play.

Interpretability focuses on understanding how the dataset can be interpreted in multiple ways. Meanwhile, explainability focuses on how this dataset can be explained to derive the correct meaning from it.

This is crucial because:

  • Users must understand the rationale behind a model's predictions to trust its output. Explainability helps build trust by providing insights into the factors influencing the model's decision-making process.
  • Understanding how a model uses features to make predictions helps ensure its generalizability to unseen data. Interpretability can reveal potential overfitting or dependence on irrelevant features, allowing us to improve the model's robustness and reliability.
  • Explainability tools can help identify hidden biases within models, ensuring they are fair and unbiased in their decision-making. This is crucial for preventing discrimination and promoting ethical AI development.

Validation vs Testing

ML model validation and model testing may seem similar. But while both validation and testing are crucial in evaluating a machine learning model's performance, they serve distinct purposes and play different roles within the model development lifecycle.

Validation is used more frequently throughout development, as it provides continuous feedback for improvement. The purpose of this is to guide model selection and, once selected, help the model fine-tune its output for accuracy and diagnose potential basses or errors.

On the other hand, testing is performed less frequently but is crucial for making final decisions about model deployment and assessing impact in real-world scenarios. This usually happens once the ML model is fully trained and optimized.

It must be tested in a real-world setting to understand how it behaves when fed with data outside its control group. Testing highlights potential weaknesses in the model and flags risks that may occur. This helps further improve the model to ensure it is equipped for real-world performance.

Best Practices in Model Validation

With a solid understanding of different validation techniques, metrics, and considerations, it's time to explore best practices for ensuring effective model validation.

By following these guidelines, you can confidently build robust and reliable models for real-world applications.

  • Choose the right validation technique based on your data and task. For this, you should consider factors like data size, distribution, and the presence of imbalanced classes.
  • Use a diverse set of metrics to evaluate performance. This helps the database understand the full range of variations and ensures that the ML model is unbiased.
  • Incorporate interpretability and explainability into your validation process.
  • Split your data carefully into training, validation, and test sets.
  • Perform validation iteratively throughout the development process.
  • Document your validation process and results clearly. This ensures transparency and facilitates the replication of your work.
  • Stay aware of potential biases and fairness issues. Utilize bias detection methods and metrics like demographic parity and equalized odds.
  • Continuously monitor and update your model over time.

Conclusion

ML model validation is the cornerstone of building reliable and trustworthy machine-learning models. By carefully evaluating a model's performance on unseen data, we can gain valuable insights into its strengths, weaknesses, and pitfalls. This allows us to make informed decisions about model deployment and ensure its effectiveness in real-world scenarios.

MarkovML empowers you to validate your ML models to achieve optimal performance effectively. Since model validation decisions often require hands-on expertise, they can lead to challenges if they are not taken with proper context and understanding. By employing responsible AI features, you can build AI solutions that are transparent, accountable, and trustworthy.

This includes:

  • The ability to identify and evaluate business risks using LLMS and classical ML models to evaluate their costs, business impact, and potential for bias. This helps you make informed decisions.
  • Unparalleled interpretability by understanding and explaining outcomes of your AI better with Connected Artifact Graph.
  • Compliance with regulations ensures that your data artefacts and AI systems comply with business regulations.

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