A validation set is a held-out dataset used after training and iterative testing to evaluate a model on data untouched by that testing process. It provides a final stopgap before treating performance as trustworthy. section 9.14
Why a third split helps
Suppose repeated model changes improve results on the test dataset. Those improvements may reflect leaked clues rather than genuine performance. Keeping a validation set separate provides a fresh check and reduces the temptation to torture the data until it produces a desired result. section 9.14


