The file S02_55.xlsx contains monthly sales (in millions of dollars) of beer, wine, and liquor. The data have not been seasonally adjusted, so there might be seasonal patterns that can be discovered. For any month in any year, define that month’s seasonal index as the ratio of its sales value to the average sales value over all months of that year.
a. Calculate these seasonal indexes, one for each month in the series. Do you see a consistent pattern from year to year? If so, what is it?
b. To “de-seasonalize” the data and get the seasonally adjusted series often reported, divide each monthly sales value by the corresponding seasonal index from part a. Then create a time series graph of both series the actual sales and the seasonally adjusted sales. Explain how they are different and why the seasonally adjusted series might be of interest.