The production manager of a chemical plant wants to determine the roles that temperature and pressure play in the yield of a particular chemical produced at the plant. From past experience, she believes that when pressure is held constant, lower and higher temperatures tend to reduce the yield. When temperature is held constant, higher and lower pressures tend to increase the yield. She does not have any idea about how the yield is affected by various combinations of pressure and temperature. She observes 80 batches of the chemical in which the pressure and temperature were allowed to vary.

a. Which model should be used? Explain.

b. Conduct a regression analysis using the model you specified in part (a).

c. Assess how well the model fits the data.