17. Split the data in Review Problem 15 into training and testing sets, using a 80:20 ratio. That is, use observations 1–202 as training data to build a regression tree using all the variables except “Index” to predict the “Percent of body fat index,” and use observations 203–252 as testing data to validate your regression tree. Report your regression tree for training data.

(a) Plot predicted values of “Percent of body fat index” versus true “Percent of body fat index” in testing data. Describe the trend.

(b) Report the correlation coefficient between the predicted values of “Percent of body fat index” versus true “Percent of body fat index,” and explain the predictability of your model.