The number of pieces of mail delivered by the U.S. Postal Service has been in steady decline since it hit a peak in 2006 with 213.1 billion. Data in the Excel file postal service.xlsx show the number of pieces of mail, in billions, delivered by the U.S. Postal Service between 1988 and 2017.

a. Construct a graph showing the number of pieces of mail delivered over time.

b. Forecast the number of pieces of mail to be delivered during 2018 using a trend projection.

c. Check the forecasting model in part b for the presence of auto-correlation using α = 0.05.

d. Comment on the appropriateness of using trend projection for these data.