Chapter 9. Judging Model Effectiveness
Once we have a model, we need to know how well it works. A quantitative approach for estimating effectiveness allows us to understand the model, to compare different models, or to tweak the model to improve performance. Our focus in tidymodels is on empirical validation; this usually means using data that were not used to create the model as the substrate to measure effectiveness.
The best approach to empirical validation involves using resampling methods that will be introduced in Chapter 10. In this chapter, we will motivate the need for empirical validation by using the test set. Keep in mind that the test set can only be used once, as explained in Chapter 5.
When judging model effectiveness, your decision about which metrics to examine can be critical. In later chapters, certain model parameters will be empirically optimized, and a primary performance metric will be used to choose the best submodel. Choosing the wrong metric can easily result in unintended consequences. For example, two common metrics for regression models are the root mean squared error (RMSE) and the coefficient of determination (a.k.a. R²). The former measures accuracy while the latter measures correlation. These are not necessarily the same thing. Figure 9-1 demonstrates the difference between the two.
A model optimized for RMSE has more variability but has relatively uniform accuracy across the range of the outcome. The right panel shows that there is a ...