For Exercises 13.18 to 13.20, see pages 670–671; for 13.21 to 13.23, see page 677; for 13.24, see page 679.

Question 13.22

13.22 Number of iPhones sold globally.

Consider data on the quarterly global sales (in millions of dollars) of iPhones from the first quarter of 2012 to the third quarter of 2014. Create indicator variables for the first, second, and third quarters along with a trend index. Call these indicator variables Q1, Q2, and Q3.

  1. Use statistical software to fit a multiple regression model of trend plus the three indicator variables. Write down the estimated trend-and-season model.
  2. Eliminate any insignificant predictor variables found in the regression output from part (a), and reestimate the trend-and-season model. Write down the estimate trend-and-season model.
  3. Based on the model found in part (b), provide forecasts of quarterly sales for the next four quarters in the future.

iphone