A rheostat knob, produced by plastic molding, contains a metal insert. The fit of this knob into its assembly is determined by the distance from the back of the knob to the far side of a pin hole. To monitor the molding operation, one knob from each hour’s production was randomly sampled and the dimension measured on each. The next table (p. 896) gives the distance measurements (in inches) for the first 27 hours the process was in operation.

a. Construct a variable control chart for the process.

b. Locate the center line, upper control limit, and lower control limit on the chart.

c. Does the process appear to be in control?

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KNOB Hour Distance Hour Distance Hour Distance Hour Distance 1 .140 8 .143 15 .144 22 .139 2 .138 9. .141 16 .140 23 .140 3 .139 10 .142 17 .137 24 .134 4 .143 11 .137 18 .137 25 .138 .142 12 .137 19 .142 26 .140 6. .136 13 .142 20 .136 27 .145 7 .142 14 .137 21 .142