The article “Ultimate Load Analysis of Plate Reinforced Concrete Beams” (N. Subedi and P. Baglin, Engineering Structures, 2001:1068–1079) presents theoretical and measured ultimate strengths (in kN) for a sample of steel-reinforced concrete beams. The results are presented in the following table (two outliers have been deleted).
Let y denote the measured strength, x the theoretical strength, and t the true strength, which is unknown. Assume that y = t + ε, where ε is the measurement error. It is uncertain whether t is related to x by a linear model t = β0 + β1x or by a quadratic model t = β0 + β1x + β2x2.
- a. Fit the linear model y = β0 + β1x + ε. For each coefficient, find the P-value for the null hypothesis that the coefficient is equal to 0.
- b. Fit the quadratic model y = β0 + β1x + β2x2 + ε. For each coefficient, find the P-value for the null hypothesis that the coefficient is equal to 0.
- c. Plot the residuals versus the lilted values for the linear model.
- d. Plot the residuals versus the fitted values for the quadratic model.
- e. c. Based on the results in parts (a) through (d), which model seems more appropriate? Explain.
- f. Using the more appropriate model, estimate the true strength if the theoretical strength is 1500.
- g. Using the mom appropriate model, find a 95% confidence interval for the true strength if the theoretical strength is 1500.
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