solution

 

. Run the model to predict corn yield using year and the squared term year2 defined in the previous exercise.

(a) Summarize the significance test results.

(b) The coefficient for year2 is not statistically significant in this run, but it was significant in the model analyzed in the previous

exercise. Explain how this can happen.

(c) Obtain the fitted values for each year in the data set and use these to sketch the curve on a plot of the data. Plot the leastsquares line on this graph for comparison. Do you see much of a difference between these two regression functions? Compute thedifference between the two predicted values for each year and plot them versus year. For what years are the differences the largest?