The accompanying data file lists monthly data on vehicle miles traveled in the U.S. (in millions). For cross-validation, let the training and the validation sets comprise the periods from Jan-12 to Dec-16 and Jan-17 to Sep-18, respectively. 

a. Use the training set to implement the Holt-Winters exponential smoothing method with additive and multiplicative seasonality and compute the resulting MSE, MAD, and MAPE for the validation set. 

b. Determine the preferred model and reimplement it with the entire data set to forecast vehicle miles for the last three months of 2018.