# Train, validation and test split discipline

Source: https://onco.cc/terms/train-test-discipline/  
OnCo record `train-test-discipline` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Data is divided into a training set the model learns from, a validation set used to choose settings, and a test set touched once at the end; using the test set to make choices turns it into a second validation set.

## Summary

In machine learning the input data used to build a model is divided into a training set, a validation set for tuning and a test set for the final, unbiased evaluation (Wikipedia). The discipline is behavioural: the test set is looked at once, patients (not samples) are the unit of splitting so one patient's slides do not appear on both sides, and the split is decided before modelling. A held-out 80/20 split is the minimum; an external cohort from another institution is the real test.

## Fields

- Kind: Term
- Last checked: 2026-09-24
- Also known as: train/validation/test split; training set; validation set; test set; held-out test set; held-out test split; 80/20 split; hold-out set
- Tags: cansim-terms

## Notes

- Listed in the CanSim terms map 1.0.0 (docs/onco/terms.json, generated 2026-09-24), CC BY 4.0, attribution: CanSim project, an open, public-data-first cancer foundation-model programme; CanSim page path /terms/train-validation-test.

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Training,_validation,_and_test_data_sets
- Wikipedia: https://en.wikipedia.org/wiki/Training,_validation,_and_test_data_sets

## Connected records

- terms: [Cancer AI vocabulary (CanSim terms map)](https://onco.cc/terms/cancer-ai-vocabulary/), [Cross-validation and stratified k-fold](https://onco.cc/terms/cross-validation/), [Data leakage in model evaluation](https://onco.cc/terms/data-leakage/), [External validation](https://onco.cc/terms/external-validation/), [Pre-registered experiment](https://onco.cc/terms/pre-registered-experiment/)

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JSON: https://onco.cc/api/v1/entities/train-test-discipline.json