Question 13.55

13.55 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. In Exercise 13.22 (page 677), you were asked to fit a trend-and-season model using regression.

iphone

  1. Use the moving-average approach to compute seasonal ratios and report their values.

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  2. Produce and plot the seasonally adjusted series. What are your impressions of this plot?
  3. Fit a trend model and report the -value of the trend coefficient. Is there enough evidence of the presence of trend?
  4. Given your conclusion of part (c), forecast iPhone sales for the fourth quarter of 2014 and for the first quarter of 2015.

13.55

(a) 1.310, 1.020, 0.811, 0.824. (b) The seasonally adjusted sales data is increasing over time. (c) . The trend term is significant, . (d) For the fourth quarter of 2014: 37.085. For the first quarter of 2015: 60.693.