数据划分与 K 折交叉验证

The problem selection creates

The three-way split

K-fold cross-validation

In a three-way split, the quantity used to compare candidate models and tune hyperparameters is the ____ error.

In $K$-fold cross-validation with $K = 5$, each round trains on ____ folds and validates on ____ fold.

A practitioner compares 300 candidate hyperparameter settings by evaluating each on the test set and picking the best. Which statement describes the consequence?