The coefficient of determination, usually written as R-squared, measures how much of the target's variance a predictive model captures. Use it as a goodness-of-fit signal, not as a complete model diagnosis. section 7.2.3
Evaluate R-squared on cross-validation predictions to see how consistently a model performs across validation subsets. Pair it with error metrics and prediction plots before deciding whether the fit is useful. section 7.2.3
Tokyo Airbnb baseline
A linear-regression baseline produced an R-squared of 0.320, with RMSE of 17,197 and MAE of 12,568 Yen. Its predictions followed a flat trend away from the ideal-fit line, indicating underfitting; a few predictions were also negative, which is infeasible for listing prices. section 7.2.3
Comparing models
For the same Tokyo Airbnb problem, XGBoost reached R-squared 0.599 while fastai reached 0.572. Comparing R-squared alongside RMSE and MAE showed XGBoost performing better across all three measures. section 12.4






