The Cherry Manufacturing Company's chief engineer examines a random sample of 10 spot welds of steel. In each case, the shear strength of the weld and the diameter of the weld are determined, the results being as follows:

a. Does the relationship between these two variables seem to be direct or inverse? Does this accord with common sense? Why or why not? Does the relationship seem to be linear?
b. Calculate the least- squares regression of shear strength on weld diameter.
c. Plot the regression line. Use this regression line to predict the average shear strength of a weld 1/5 inch in diameter. Use the regression line to predict the average shear strength of a weld 1/4 inch in diameter.

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Shear strength (pounds) Weld diameter (thousandths of an inch) 680 800 780 885 975 1,025 1,100 1,030 1,175 1,300 190 200 209 215 215 215 230 250 265 250