A textbook publishing company has compiled data on total annual sales of its business text-books for the preceding eight years:

a. Plot the data and fit an appropriate model to it. Forecast the preceding eight years, and determine the forecast errors. Finally, construct a 2s control chart.
b. Using the model, forecast textbook sales for each of the next five years.
c. Suppose actual sales for the next five years turn out as follows:

Calculate tire forecast errors for years 9 to 13. Is the forecasting process in control? Explain.