Eighteen plots of land in Indiana were given different applications of nitrogen. In each case, the previous crop had been soybeans and all other factors were roughly comparable. Use the 18 observations in Table 9.16 to estimate the equation Y = α + βX + ε, where Y is the corn yield (bushels per acre) and X is nitrogen (pounds per acre). Is the relationship substantial and statistically significant at the 1 percent level?

Now make a scatter diagram with the residuals on the vertical axis and X on the horizontal axis and a horizontal line drawn at 0. What pattern do you see and how do you explain it?

Does this pattern make logical sense?

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Nitrogen Yield Table 9.16: Exercise 9.47 Nitrogen Yield Nitrogen Yield 50 91 90 129 170 160 130 155 250 175 250 183 90 138 210 185 170 177 170 172 130 170 90 124 130 155 250 166 50 95 210 183 50 84 210 171