This is the author speaking here. Did you solve the mystery of the revenues drop? You did not, I guess. Nevertheless, you made some relevant steps on your journey to learning how to use R for data mining activities.
In this chapter, you learned some conceptual and some practical stuff, and you now possess medium-level skills to define and measure the performance of data mining models.
Andy first explained to you what we do intend for model performance and how this concept is related to the one of model interpretability and the purposes for which the model was estimated.
You then learned what the main model metrics are for both regression and classification problems.
Firstly, you were introduced to the relevant concepts of error, mean squared errors, and R-squared.
About this latter statistics, you also carefully analyzed its meaning and the common misconceptions...
