The accompanying data file contains weekly data on crew size (the number of workers) and productivity (jobs/week) over the past 27 weeks. A linear regression model and a quadratic regression model are considered for predicting productivity on the basis of crew size. 

a. Use the holdout method to compare the predictability of the models using the first 18 observations for training and the remaining 9 observations for validation. 

b. Use the k-fold cross-validation method to compare the predictability of the models using k = 3.