Tire classified department of a monthly magazine has used a combination of quantitative and judgmental methods to forecast sales of advertising space. The forecast errors over an 18-month period are as follows:

a. Using tire first half of tire data (Months 1 to 9), construct a control chart with 2s limits.
b. Plot the last nine forecast errors on the control chart. Is the forecasting process in control? Are tire forecast errors random? Explain.