The unit sales of a new drug for the first 25 months after its introduction to the marketplace are recorded in the file P12_15.xlsx.
a. Estimate a linear trend equation using the given data. How well does the linear trend fit these data? Are the residuals from this linear trend model random?
b. If the residuals from this linear trend model are not random, propose another regression-based trend model that more adequately explains the long-term trend in this time series. Estimate the alternative model(s) using the given data. Check the residuals from the model(s) for randomness. Summarize your findings.
c. Given the best estimated model of the trend in this time series, interpret R2 and se.