Assume that the following demands vary according a four-period seasonal cycle:

Month: 1 2 3 4 5 6 7 8 

Demand: 25 34 39 21 54 73 89 59 

a. Compute the seasonal indexes using the average demand in each cycle as the base. Then compute the average index for each "season" across the two cycles. 

b. Compute the seasonal indexes using regression estimates as the base. Then compute the average index for each "season" across the two cycles.