Immediately after training, a model scores 99% accuracy on its training data but only 71% on a held-out test set drawn from the same source and time period. Which problem does this indicate?
Choose one.
Overfitting: a model fits its training data too closely, including noise, and generalizes poorly to new data.
The deciding signal is the gap: near-perfect on training data, much worse on unseen data from the same distribution. That is overfitting. Data drift is the tempting distractor because performance on new data is poor, but drift happens over time in production as real-world data changes; here the test data is from the same source and period and the gap shows up immediately. Underfitting would be poor on the training data too.
- Compare training performance with performance on unseen data.
- A large gap with strong training results points to overfitting.
- Rule out drift by noting the test data matches the training distribution and time.
Exam tip: Great on training data, poor on new data from the same distribution means overfitting.
AI vs ML vs Deep Learning: Core AIF-C01 Concepts and Terminology — the lesson that teaches this.