Underfitting occurs when a model is too simple to capture important patterns in the data. It often appears as weak predictive performance and systematically flat predictions. section 7.2.3
Linear regression baseline
A linear regression baseline for listing prices produced an R-squared of 0.32, MAE above 12,000 Yen, and predictions that departed from the ideal-fit line with a flat trend. Some predictions were negative, although only about 0.5% were nonpositive. These results signal that the baseline misses important structure rather than providing a strong final model. section 7.2.3
Use baseline diagnostics to investigate whether the model is underfitting:
- Compare cross-validation predictions with expected target values across data subsets. section 7.2.3
- Inspect feature coefficients and individual case contributions before choosing a more complex model. section 7.4
- Use the baseline’s limitations to guide tentative choices for nonlinearities, interactions, targets, and hyperparameters. section 7.4






