A surrogate model is an approximate model of an objective function. In Bayesian optimization, it predicts model performance across untested hyperparameter settings, reducing the need to evaluate every combination directly. section 6.3.4
A practical Bayesian optimization loop works as follows:
- Build or update the surrogate from feedback produced by the objective function. section 6.3.4
- Use an acquisition function to balance exploration of uncertain settings with exploitation of promising settings. section 6.3.4
- Choose the next hyperparameters, evaluate the model, and repeat the fine-tuning cycle. section 6.3.4
Tuning an XGBoost classifier
An Optuna study can search settings such as learning rate, tree depth, subsampling, and regularization, then report the best parameters and validation score after 60 trials.




