Mobile Africa These data give the annual number of mobile phone subscribers in sub-Saharan Africa in 2000–2015.

(a) Fit a polynomial trend model to the number of phone subscribers. Use the variable Year in the data set as the time index in these models. Explain your choice of the best of these models.

(b) Replace the time index Year in your preferred model by Year-2000 (the number of years since 2000). How does this change alter the estimated model?

(c) What does your chosen model indicate about the rate of growth of mobile phone users in Africa?

(d) Use your preferred model to generate a prediction interval for the number of subscribers in 2020. Do you think the interval is reliable?