Compute an approximate 95 percent prediction interval forecast of tractor sales for each of the quarters next

Question:

Compute an approximate 95 percent prediction interval forecast of tractor sales for each of the quarters next year. Use the fact that the half-lengths of 95 percent prediction intervals for the deseasonalized sales values in the four quarters of next year are, respectively, 14, 14.4, 14.6, and 15.

International Machinery, Inc., produces a tractor and wishes to use quarterly tractor sales data observed in the last four years to predict quarterly tractor sales next year. The following MegaStat output gives the tractor sales data and the estimates of the seasonal factors and trend line for the data:

Centered Sales, y Deseasonalized Ratio to Seasonal Moving Average Sales, y Indexes Year Quarter CMA 293 1.191 245.9 392