Department Store Sales. The time plot in Figure $18\mathrm{.}7$ describes actual quarterly sales for a department store over a $6-$year period $($data are available in DepartmentStore$-$Sales.csv$,$ data courtesy of Chris Albright$).$

The forecaster was tasked to generate forecasts for $4$ quarters ahead. He therefore partitioned the data such that the last $4$ quarters were designated as the validation period. The forecaster approached the forecasting task by using multiplicative Holt$-$Winter's exponential smoothing. The smoothing parameters used were a $=0\mathrm{.}2,$ B $=0\mathrm{.}15,7=0\mathrm{.}05.$

i$.$ Run this method on the data.