13.36 Metal Corrosion and Soil Acids In an investigation to determine the relationship between the degree of metal corrosion and the length of time the metal is exposed to the action of soil acids, the percentage of corrosion and exposure time were measured weekly. 

The data were fitted using the quadratic model, E(Y) Bo+Bx+Bax, with the following results.

a. What percentage of the total variation is explained by the quadratic regression of Y on x?

b. Is the regression on x and x significant at the a = 0.05 level of significance?

c. Is the linear regression coefficient significant when x2 is in the model?

d. Is the quadratic regression coefficient significant when x is in the model?

e. The data were fitted to a linear model without the quadratic term with the results that follow. What can you say about the contribution of the quadratic term when it is included in the model?

f. The plot of the residuals from the linear regression model in part e shows a specific pattern. What is the term in the model that seems to be missing?

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y 0.1 0.3 0.5 0.8 1.2 1.8 2.5 3.4 x 1 2 3 4 5 6 7 8