The relationship between yield of maize, date of planting, and planting density was investigated in the article “Development of a Model for Use in Maize Replant Decisions” (Agronomy Journal [1980]: 459–464). Let y = percent maize yield x1 = planting date 1days after April 202 x2 = planting density 110,000 plants/ha2 The regression model with both quadratic terms (y = a
a. If a = 21.09, b1 = .653, b2 = .0022, b3 = 2.0206, and b4 = 0.4, what is the population regression function?
b. Use the regression function in Part (a) to determine the mean yield for a plot planted on May 6 with a density of 41,180 plants/ha.
c. Would the mean yield be higher for a planting date of May 6 or May 22 (for the same density)?
d. Is it legitimate to interpret b1 = .653 as the average change in yield when planting date increases by one day and the values of the other three predictors are held fixed? Why or why not?

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