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Commonly confused · AI & Machine Learning

Overfitting vs Underfitting: What's the Difference?

Overfitting happens when a model learns the training data too closely, including noise, so it performs well on training data but poorly on new data. Underfitting happens when a model is too simple to capture the underlying pattern, so it performs poorly on both.

Overfitting

Full definition
vs

Underfitting

Full definition
OverfittingUnderfitting
Training performanceVery goodPoor
Validation/test performancePoorPoor
Bias / varianceLow bias, high varianceHigh bias, low variance
Typical causeToo complex model, too little data, too many epochsToo simple model, too few features, too much regularization
FixesMore data, regularization, dropout, early stopping, simpler modelMore features, more complex model, train longer, less regularization
Remember it

Overfitting memorizes the textbook; underfitting never opened it.

Exam tip

A large gap between training and validation accuracy signals overfitting. Exams pair overfitting with high variance and underfitting with high bias.

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