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

a$.$ Which of the following methods would not be suitable for forecasting this series?

$$ Moving average of raw series

$$ Moving average of deseasonalized series

$$ Simple exponential smoothing of the raw series

$$ Double exponential smoothing of the raw series

$$ Holt$-$Winter's exponential smoothing of the raw series

b$.$ The forecaster was tasked to generate forecasts for four quarters ahead. He therefore partitioned the data such that period, fourquaster apedeen ted are sig task by

using multiplicative Holt$-$Winter's exponential smoothing.

The smoothing parameters used were a $=0\mathrm{.}2,$ B $=0\mathrm{.}15,$ V$=$

i$.$ Run this method on the data.

ii$.$ The forecasts for the validation set are given in Table

$18\mathrm{.}3.$ Compute the MAPE values for the forecasts of

Q$21$ and Q$22.$