Reconsider the mine fracture data from Problem 13.7. Remove any regressors from the original model that you think might be unimportant and rework parts b-e of Problem 13.7. Comment on your findings.

Data From Problem 13.7

Myers [1990] presents data on the number of fractures (y) that occur in the upper seams of coal mines in the Appalachian region of western Virginia. Four regressors were reported: x1=${\mathrm{????}}_1=$ inner burden thickness (feet), the shortest distance between seam floor and the lower seam; x2=${\mathrm{????}}_2=$ percent extraction of the lower previously mined seam; x3=${\mathrm{????}}_3=$ lower seam height (feet); and x4=${\mathrm{????}}_4=$ time (years) that the mine has been in operation. The data are shown below.

Observation Number of Fractures per Subregion, y x X2 X3 X4 10 1234567820 9 11 12 13 14 15 16 17 22222222 18 19 20 21 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 201412004ALAL52555OSTI3322O1523330020032350m 50 70 230 65 125 70 75 65 2453 1.0 6.9 1.0 68 0.5 70 65 53 0.5 65 70 46 3.0 65 60 62 1.0 350 60 54 0.5 350 90 54 0.5 160 80 38 0.0 145 65 38 10.0 145 85 38 0.0 180 70 42 2.0 43 80 40 0.0 42 85 51 12.0 42 85 51 0.0 45 85 42 0.0 83 85 48 10.0 300 65 68 10.0 190 90 84 6.0 145 90 54 12.0 510 80 57 10.0 65 75 68 5.0 470 90 90 9.0 300 80 165 9.0 275 90 40 4.0 420 50 44 17.0 65 80 48 15.0 40 75 51 15.0 900 90 48 35.0 95 88 36 20.0 40 85 57 10.0 140 90 38 7.0 150 50 44 5.0 80 60 96 5.0 80 85 96 5.0 145 65 72 9.0 100 65 72 9.0 150 80 48 3.0 150 80 48 0.0 210 75 42 2.0 11 75 42 0.0 100 65 60 25.0 50 88 60 20.0