Generalization is a model’s ability to make appropriate predictions on new data rather than merely memorizing the observations used for fitting. section 8.5
A fitted model is not automatically useful: its performance must be checked on data it has not seen. Train/test splits and cross-validation withhold data for later validation. section 8.5 section 1.8
A classification check
A logistic regression model might fit its training observations perfectly by memorizing them. Testing it on withheld observations reveals whether its learned pattern extends beyond the training data. section 8.5












