Introduction
Cross-validation tells you how good a model is. Learning curves tell you why it's not better, and what to do about it — in particular, the expensive question: "Will collecting more data help?"
A learning curve is like tracking a student's scores as they study more practice questions:
- If both practice and exam scores stay low however much they study, the student needs a better method (more capable model), not more questions.
- If practice scores are perfect but exam scores lag far behind, they're memorising — more varied practice questions (more data) and less cramming (regularization) will help.