The accompanying data file lists quarterly data on median house prices in the West Census region. For cross-validation, let the training and the validation sets comprise the periods from 2010:01 to 2016:04 and 2017:01 to 2018:03, 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 house price for the fourth quarter of 2018.