38. The accompanying data resulted from an experiment to study the nature of dependence of welding current on the three factors: welding voltage, wire feed speed, and tip-to-workpiece distance. There were two levels of each factor (a 23 experiment) with two replications per combination of levels (the averages across replications agree with values given in the article “A Study on Prediction of Welding Current in Gas Metal Arc Welding,” J. Engr. Manuf., 1991: 64–69). The first two given numbers are for the treatment (1), the next two for

a, and so on in standard order: 200.0, 204.2, 215.5, 219.5, 272.7, 276.9, 299.5, 302.7, 166.6, 172.6, 186.4, 192.0, 232.6, 240.8, 253.4, 261.6.

a. Verify that the sums of squares are as given in the accompanying ANOVA table from MINITAB.

b. Which effects appear to be important, and why?

Analysis of Variance for current Source DF SS MS F P Volt 1 1685.1 1685.1 102.38 0.000 Speed 1 21272.2 21272.2 1292.37 0.000 Dist 1 5076.6 5076.6 308.42 0.000 Volt*speed 1 36.6 36.6 2.22 0.174 Volt*dist 1 0.4 0.4 0.03 0.877 Speed*dist 1 109.2 109.2 6.63 0.033 Volt*speed*dist 1 23.5 23.5 1.43 0.266 Error 8 131.7 16.5 Total 15 28335.3