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Training, Validation and Test Sets: How To Split Machine Learning Data

A concise explanation of the differences between ML training, validation and test sets | How to include enough data to train machine learning models.

Training, Validation and Test Sets: How To Split Machine Learning Data

In machine learning (ML), a fundamental task is the development of algorithm models that analyze scenarios and make predictions. During this work, analysts fold various examples into training, validation, and test datasets. Below, we review the differences between each function.

Train vs. Validate vs. Test

Training datasets comprise samples used to fit machine learning models under construction, i.e., carry out the actual AI development. Constructing these robust pillars of AI involves following best practices.

Training data are collections of examples or samples that are used to 'teach' or 'train the machine learning model. The model uses a training data set to understand the patterns and relationships within the data, thereby learning to make predictions or decisions without being explicitly programmed to perform a specific task.

In contrast, validation datasets contain different samples to evaluate trained ML models. It is still possible to tune and control the model at this stage. Working on validation data is used to assess the model performance and fine-tune the parameters of the model. This becomes an iterative process wherein the model learns from the training data and is then validated and fine-tuned on the validation set. A validation dataset tells us how well the model is learning and adapting, allowing for adjustments and optimizations to be made to the model's parameters or hyperparameters before it's finally put to the test.

Finally, a test data set is a separate sample, an unseen data set, to provide an unbiased final evaluation of a model fit. The inputs in the test data are similar to the previous stages but not the same data. The test data set mirrors real-world data the machine learning model has never seen before. Its primary purpose is to offer a fair and final assessment of how the model would perform when it encounters new data in a live, operational environment.

The training data and validation data can play additional roles in model preparation. They can aid in feature selection, a process in which the most relevant or significant variables in the data are selected to improve the model performance. They can also contribute to tuning the model's complexity, balancing fitting the data well, and maintaining a good level of generalization. The fit of the final model is a combined result from the aggregate of these inputs.

Training Data Sets

Initially, machine learning development involves starting inputs within specified project parameters. These parameters could include the specific problem the model is designed to solve, the type of data it will process, the performance metrics it aims to optimize, etc.

The process of training machine learning models involves setting up a complex network of connections between the individual elements within the model. These individual elements are often referred to as 'neurons' in the context of neural networks.

One of the critical aspects requires the expert setting of weightings between the various connections of so-called neurons within the ML model or estimator*. However, these initial settings are not static; they are modified during the training process based on the feedback received from the model performance.

After introducing this first set of training data, developers compare the resulting output to target answers. This comparison forms the basis for an error or loss function that quantifies the model performance - the discrepancy between the predictions and the actual values. Next, they adjust the model's parameters, weighting, and functionality. This adjustment is done using various optimization algorithms, the most common of which is Gradient Descent.

More than one 'epoch' or iteration of this adjustment loop is often necessary. The ultimate goal is to create models that can make accurate predictions when exposed to new, unknown data that they weren't trained on. To ensure that the machine learning model can generalize well, one must strike a balance during the training process to avoid underfitting (where the model is too simple to capture the underlying pattern) or overfitting (where the model is overly complex and captures the noise in the training data).


Validation Data Sets

The next stage involves using a validation set to estimate the accuracy of the ML model concerned. During this phase, developers ensure that new data classification is precise and results are predictable.

Validation sets comprise unbiased inputs and expected results designed to check the function and performance of the model. Cross-validation (CV) techniques come into play during this phase, and there are different methods of CV that exist, but all aim to ensure stability by estimating how a predictive model will perform. An example is the usage of rotation estimation or out-of-sample testing to assure reasonable precision.

Resampling and fine-tuning involve various iterations. Whatever the methodology, these verification techniques aim to assess the results and check them against independent inputs. It is also possible to adjust the hyperparameters, i.e., the values used to control the overall process.

That said, not all models require validation sets. Some experts consider that ML models with no hyperparameters or those that do not have tuning options do not need a validation set. Still, in most practical applications, validation sets play a crucial role in ensuring the model's robustness and performance.

Test Data Sets

The final step is to use a test set to verify the model's functionality. Some publications refer to the validation dataset as a test set, especially if there are only two subsets instead of three. Similarly, if records in this final test set have not formed part of a previous evaluation or cross-validation, they might also constitute a holdout set.

Test samples provide a simulated real-world check using unseen inputs and expected results. In practice, there could be some overlap between validation and testing. Each procedure shows that the ML model will function in a live environment once out of testing.

The difference is that while validating, the results provide metrics as feedback to train the model better. In contrast, the performance of a test procedure merely confirms that the model works overall, i.e. as a black box with inputs passed through it. During this final evaluation, there is no adjustment of hyperparameters.

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How to Split Your Machine Learning Data

Above, we have seen the distinction between the different types of sets. The next decision is splitting enough data between each of them.

The optimum ratio when dividing records with enough data between each function – train, validate, and test – depends on the application usage, model type, and data dimensions. Most ML models benefit from having a substantial number of scenarios from which to train.

At the validation stage, models with few or no hyperparameters are straightforward to validate and tune. Thus, a relatively small dataset should suffice.

In contrast, models with multiple hyperparameters require enough data to validate likely inputs. Cross-validation might be helpful in these cases, too. Generally, apportioning 80 percent of the data to train, 10 percent to validate, and 10 percent to test scenarios ought to be a reasonable initial split.

Common Pitfalls in The Training Data Split

However, a validation dataset must not be too small. Otherwise, the ML model will be untuned, imprecise, or even inaccurate. In particular, the F1 score – a statistical measure of precision and recall – will vary too widely.

One cycle through a complete dataset in artificial neural networks is called an epoch. And as mentioned earlier, training a model usually takes more than one epoch.

The train-test-validation ratio depends on the usage case. Getting the procedure right comes with experience.


An alternative approach involves splitting an initial dataset into two halves, training and testing. Firstly, with the test data set kept to one side, a proportion of randomly chosen training data becomes the actual training set. The remaining values in the array are for later iterations to validate inputs. For example, the split might vary from two halves to a ratio of 80:20 percent.

This cross-validation involves one or more splits of the training data set and validation data set. In particular, K-fold cross-validation aims to maximize accuracy in testing by dividing the source data into several bins or groups. All except one of these are for training and validation purposes. The last is for testing.

In this method, each data set runs as a separate experiment. Analysts then calculate the average of all the runs to obtain the mean accuracy. Once the result falls within specified limits, the final step before signoff is using the remaining fold of test data to double-check the findings.

Other methods of cross-validation are Stratified K-Fold cross-validation to guarantee a suitable representation of each class and avoid bias, Leave-P-Out cross-validation, which is ideally used for smaller sample sizes because of its high computational demands, and Rolling cross-validation for time series-based data.

Low-Quality Training Data

Like other areas of IT, machine learning algorithms follow the time-tested principle of GIGO: garbage in, garbage out. So, to ensure reliable and robust algorithms, the following three components are necessary:

  • Quantity. Sufficient data is important for the model to learn how to interact with users. As an analogy, humans need a considerable amount of information before becoming an expert.

  • Quality. In themselves, data will not guarantee reliable results. Real-world scenarios and test cases that represent likely conditions are vital. Data should mimic the user input that the new algorithm will receive. It is essential to fold in data on which the application will rely, such as a combination of images, videos, sounds, and voices.

  • Diversity. ML requires algorithms trained on more than one input fold to simulate most, if not all, likely and possible cases.

Designers should seek to prevent bias in models. Applications must comply with legislation and should conform to inclusivity guidelines. They should not display prejudice based on age, race, gender, language, marital status, or other identifying factors.


When developing an ML model, an essential principle is that the validation set and test set must remain separate. Otherwise, an overfit might occur. Overfitting is when exceptional or unreal conditions lead to incorrect outputs. In other words, the statistical model fits precisely with the inputs used to train it and will probably not be accurate.

Instead, the aim should be to generalize. The proper application of cross-validation ought to minimize overfitting and ensure that the algorithm's prediction and classification functionality are correct.

Overemphasis on Validation and Test Set Metrics

Overfitting can also arise if the development methodology overuses search techniques to look for empirical relationships in the input samples. This approach tends to identify relationships that might not apply in the real world.

As an analogy, it is akin to looking for connections that do not exist between random events. Nonetheless, discerning between occasional coincidences and the emergence of new patterns does involve a delicate balancing act, with careful evaluation of the probabilities involved.

Although a validation set contains different data, it is essential to remember that evaluations should not be too lengthy. Otherwise, the model tends to become more biased as validation data perfuses into the configuration.

Training, Validation & Test Sets: Key Takeaways

Quality is paramount for AI to deliver accurate results in the ingenious and expanding field of ML. Sound predictions and stable system behavior require the correct application of various key principles.

Essential considerations when organizing data for test sets are that:

  • Training data builds the ML algorithm. Analysts feed in information and look for corresponding expected outputs. The import should be within specification and sufficient in quantity.

  • When validating, it is necessary to use fold-in unseen values. Here, the new inputs and outputs enable ML designers to evaluate how accurate the new model's predictions are.

  • Overfitting can result from too much searching or excessive amounts of validation data.

  • While supervised ML approaches use a tag or classifier to identify training records, test records must remain untagged. The same data labels could enable the ML model to single out a shared reference, leading to anomalies in the results.

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FAQ on Training, Validation, and Test Sets

How can data labeling help with model training?

High quality data labeling is crucial when it comes to model training. Training, validation, and test sets should be accurate to ensure that developers build an effective machine learning model efficiently. Accurate training data helps the model learn the right patterns, validation data helps developers fine-tune the model correctly, and test data provides trustworthy metrics so they can confidently deploy their AI solution.

What are training validation and test data sets?

Training, validation, and test data sets are three subsets of data used in machine learning.

  • Training data set: This is the largest subset used to train the model by adjusting its parameters. It helps the model learn the underlying patterns in the data.

  • Validation data set: We use this set to provide an unbiased evaluation of the model during the training phase. It is used to fine-tune the model's parameters and select the best-performing model.

  • Test data set: This set provides an unbiased evaluation of the final model. After the model has been trained and validated, the test set is introduced to check the model's performance on completely unseen data.

Do I need a test set if I have a validation set?

Yes, you still need a test set, even if you have a validation set. The validation set is for tuning the model parameters, and it can indirectly influence the model during the training process, potentially leading to overfitting. A test set is necessary to evaluate the model's performance on entirely new data, providing an unbiased assessment of how the model would perform in a real-world application.

What is the difference between a validation set and a test set?

The validation set is used during the training phase of the model to provide an unbiased evaluation of the model's performance and to fine-tune the model's parameters. The test set, on the other hand, is used after the model has been fully trained to assess the model's performance on completely unseen data. Unlike the validation set, no tuning or adjustment is made based on the test set.

What is the difference between a test set and a training set?

The training set trains the machine learning model, allowing it to learn the patterns and relationships within the data. The test set used after the model has been trained and validated, to provide an unbiased evaluation of the model performance on completely new, unseen data. The training set influences the model directly during the learning process, while the test set does not influence the model and is only used for model performance evaluation.

What is cross-validation, and how can it help in training a more accurate model?

Cross-validation is a robust technique in machine learning used to assess a model's predictive performance and ensure it is not overfitting the training data. By partitioning the data set into several subsets, training occurs on a subset while validation and tuning parameters happens on the remaining validation data and finally, testing on the test data. This process repeats several times with different partitions, improving the model's overall accuracy by providing a comprehensive training and validation process. It's a crucial step in creating a high-performing, generalizable machine learning model.

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