Hold-out validation reserves part of a dataset, trains on the remainder, and evaluates on the reserved data. Keep a separate, untouched test set when tuning models. section 5.2.1
A simple split
Shuffle the data, set aside a validation slice, train on the remaining samples, and evaluate on the slice. After choosing the model, retrain with training and validation data combined, then measure generalization once on the untouched test set. section 5.2.1
Use these practical checks:
- Treat unstable scores across different random splits as a warning that the validation or test sets are not representative. section 5.2.1
- Prefer K-fold or iterated K-fold validation when limited data makes a single split unreliable. section 5.2.1

