What is the purpose of a Validation Set?

What is the purpose of a Validation Set?

Approach: Explain the middle step between training and final testing.

A subset of the dataset used to tune hyperparameters and provide an unbiased evaluation of a model fit while tuning. It prevents 'leaking' the test set information into the training process.

Follow-up: How does it differ from the Test Set?

Key concepts: tuning, holdout, unbiased evaluation.