- Overfitting — great training accuracy, poor test accuracy; the model is too complex relative to the data
- Underfitting — poor accuracy on both train and test; the model is too simple to capture the pattern
- Fixes for overfitting — more data, regularization, simpler model, dropout, early stopping
- Fixes for underfitting — more features, more complex model, less regularization
Tip
Draw the classic 'training vs validation loss curve diverging' picture in your head — it's the mental model interviewers expect.