During oil drilling operations, components of the drilling assembly may suffer from sulfide stress cracking. The article

“Composition Optimization of High-Strength Steels for Sulfide Cracking Resistance Improvement” (Corrosion Science, 2009: 2878–2884) reported on a study in which the composition of a standard grade of steel was analyzed. The following data on and was read from a graph in the article

(which also included the equation of the least squares line).

x 635 644 711 708 836 820 810 870 856 923 878 937 948 y 100 93 88 84 77 75 74 63 57 55 47 43 38

a. What proportion of observed variation in stress can be attributed to the approximate linear relationship between the two variables?

b. Compute the estimated standard deviation .

c. Calculate a confidence interval using confidence level 95% for the expected change in stress associated with a 1 MPa increase in strength. Does it appear that this true average change has been precisely estimated?