Exercises 11.98 through 11.104 use the CROPS data file, which contains the U.S. yield (bushels/acre) of corn and soybeans from 1957-2013.23

Question 11.102

11.102 Compare models.

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

crops

  1. Summarize the significance test results.
  2. The coefficient for year2 is not statistically significant in this run, but it was highly significant in the model analyzed in the previous exercise. Explain how this can happen.
  3. 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 least-squares line on this graph for comparison. Describe the differences between the two regression functions. For what years do they give very similar fitted values? For what years are the differences between the two relatively large?