Estimate the S × I portion of the multiplicative model by finding the ratio-to-moving-averages for the time-series data. Determine whether these ratio-to-moving-averages are stable from year to year.
Store, which has maintained monthly sales records for the past 48 months, with the following results:

1457(6) 4( 855 S( 2 7 7 4 6 4 35 4 4 3 3 2 3 3 3 3 3 3 3 3 1-901 678 7 6 933 1469 () 1 2 3 4 5 6 7 8 Month Sales (S) 32.400 35,600 31,200 34.600 36.800 35,700 Month Sales (S) 37.500 40,000 43.200 46.700 50.100 52,100 37 (Jan) 38 39 43 46 48