24. Textbook Sales. The quarterly sales data (number of copies sold) for a college text- book over the past three years are as follows: Year 1 1 1 2 2 2 2 3 3 3 3 Quarter 1 2 3 4 1 2 3 4 1 2 3 4 Sales 1,690 940 2,625 2,500 1,800 900 2,900 2,360 1,850 1,100 2,930 2,615 a. Construct a time series plot. What type of pattern exists in the data? b. Use a regression model with dummy variables as follows to develop an equation to account for seasonal effects in the data: Qtr1 = 1 if quarter 1, 0 otherwise; Qtr2 = 1 if quarter 2, 0 otherwise; Qtr3 = 1 if quarter 3, 0 otherwise. c. Based on the model you developed in part (b), compute the quarterly forecasts for the next year. d. Let 1 refer to the observation in quarter 1 of year 1; 1 = 2 refer to the observa- tion in quarter 2 of year 1; ...; and r = 12 refer to the observation in quarter 4 of year 3. Using the dummy variables defined in part (b) and 1, develop an equation to account for seasonal effects and any linear trend in the time series. e. Based upon the seasonal effects in the data and linear trend, compute the quarterly forecasts for the next year. f. Is the model you developed in part (b) or the model you developed in part (d) more effective? Justify your answer.