What is Underfitting?

Underfitting occurs when a model fails to capture the complexity of the training data and thus is a poor representation of the relationship between the features and the target. For example, if the actual relationship between a predictor and the target is quadratic, but the model is specified with only a linear term, it would be an incorrect model for the actual relationship. Models that are underfit are not useful, since they fail to capture the appropriate nature of the relationship. Underfitting can occur due to misspecification or lack of tuning of the appropriate hyperparameters.