What is Splitting Data for Machine Learning Models?


Machine learning has revolutionized various industries, empowering them with predictive analytics and intelligent decision−making. However, before a machine can learn, it needs data to train on. One crucial step in the machine learning pipeline is splitting the available data into different subsets for training, validation, and testing purposes. This article explores what exactly is meant by splitting data for machine learning models and why it's essential for model performance.

Splitting Data for Machine Learning Models

For most conventional machine learning tasks, this involves creating three primary subsets: training set, validation set (optional), and test set. In essence, data splitting refers to dividing an available dataset into distinct groups based on specific criteria or proportions.

  • Training Set:

    The training set constitutes the largest portion of the original dataset and serves as fuel for our model's learning phase. We present this labeled data to our algorithm during its training stage so that it can understand patterns within features and their corresponding labels accurately.

    Training set (70%): Used by the model to learn patterns from available data.

  • Validation Set

    While not always mandatory, a validation set aids in fine−tuning our models by assessing their performance during hyperparameter tuning or feature selection processes. It helps us avoid overfitting − when a model performs well on known examples but poorly generalizes to new ones.

    Validation set (15%): Employed during training to tune hyperparameters and prevent overfitting.

  • Test Set:Finally comes the critical test set−an unseen subset used exclusively at the very end once we've trained our model completely−its purpose being evaluating how well it generalizes with real−world scenarios beyond just memorizing information from training samples.

    Test set (15%): Reserved for final evaluation purposes after model refinement.

Significance of Data Splitting

Effective data splitting plays several pivotal roles in achieving reliable machine learning models:

  • Performance Estimation: By evaluating predictions on an independent test set untouched during development stages allows us to estimate how accurate and robust our final model truly is when presented new data points.

  • Model Selection: Validation sets help compare multiple potential models' performances and select the one with optimal hyperparameters, architectures, or feature sets. This prevents overfitting by assessing generalization capabilities before deployment.

  • Avoiding Leakage: Splitting data separates informative patterns embedded purely within training samples from those found in validation or test sets. It ensures our models learn valuable insights while avoiding accidental learning of information specific to these subsets rather than underlying relationships inherent in the problem.

Best Practices for Data Splitting

To ensure fair evaluation and avoid biased results when splitting your data, it's essential to follow some key guidelines:

  • Randomness: Use proper random sampling techniques to split datasets randomly so that they are representative of real−world scenarios.

  • Stratification: If dealing with imbalanced classes (e.g., fraud detection), maintain class distribution proportions across splits by using stratified sampling methods.

  • Proper Sizing: There is no fixed rule for set sizes; appropriate selection depends on various factors like dataset size, complexity, and available computing resources. However, common practices suggest tasks being 70−80% training set, up to 15−20% validation set (if used), and remaining 10−20% as a test set.

Methods for splitting data

Random Sampling Method

The random sampling method is one of the most used techniques for data splitting. It randomly divides the dataset into training, validation, and testing sets with defined proportions such as 70−15−15 or 80−10−10.

This technique is popular due to its simplicity but may suffer from potential bias if randomness happens to disproportionately separate certain patterns or features unique to specific subsets.

Stratified Sampling Technique

Stratified sampling addresses biases introduced by simple random sampling when working with imbalanced datasets. By maintaining class distribution proportions while generating subsets, stratified sampling ensures that all classes are adequately represented in each subset's population.

For example, in a binary classification problem where Class A constitutes 90% of samples while Class B represents just 10%, stratified sampling would proportionally distribute both classes within each subset−thus reducing class imbalance−related issues during model evaluation.

Time−Based Splitting

Time−based splitting considers temporal ordering while creating partitions within time−series datasets−a common scenario in financial predictions or weather forecasting applications−where older observations might not hold significance compared to recent ones.

In time−based partitioning, the data is sorted chronologically, with a designated cut−off point separating earlier observations for training or validation and more recent observations for testing. This approach leverages temporal dependencies while ensuring generalizability to future time instances.

K−Fold Cross−Validation

It mitigates potential weaknesses of traditional random or stratified samplings by utilizing iterative resampling techniques. The dataset is divided into 'k' equal−sized folds (typically 5 or 10), where each fold acts as both the validation set once and part of the training set 'k−1' times.

This technique enables more robust model evaluation as it minimizes chances of overfitting or underfitting while also maximizing data utilization for learning and assessing performance across various subsets.


Data splitting is an essential prerequisite for successful machine learning modeling. By utilizing advanced techniques like K−fold cross−validation, stratified splitting, or time−series splitting alongside Python's powerful libraries such as scikit−learn (sklearn), researchers can optimize their models' performance effectively. Splitting our data properly is crucial for building robust machine learning models.

Updated on: 28-Jul-2023


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