Didn't understand: (Model selection based on degree of polynomial) Overly optimistic regards to generalization error

Here is a visual example for overfitting: you can see that the overfitting model on the right is just too complex and it is oscillating too much in certain areas. These are the areas the model does not generalise sufficiently and might cause big issues:

Source: https://kluedo.ub.rptu.de/frontdoor/deliver/index/docId/6867/file/_Karatas-Comparison_of_model_based_methods_with_machine_learning_strategies-3.pdf

Especially high order polynomial fits can suffer from this oscillating / overshooting, especially in the edge regions, see also:


Please let me know if this answers your question, @tennis_geek.

Have a good one! :tennis:

Best regards