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 y = threshold stress (% SMYS) and x = yield strength (MPa) was read from a graph in the article (which also included the equation of the least squares line).

(xi = 10,576, (yi = 894, (x2i = 8,741,264,
(y2i = 66,224, (xiyi = 703,192
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 S 1.
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?

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x | 635 644 711 708 836 820 810 870 856 923 878 937 948 y100 93 88 84 77 75 74 63 57 55 47 43 38