Statistical Quality Control Copper wires are coated with a thin plastic coating. Samples of four wires are taken every hour, and the thickness of the coating (in mils) is measured. The means are X = 150.075, R=6.97 and s = 3.082 a. Compute the 3s limits for the R chart. Is the variance out of control at any point? If so, delete the samples that are out of control and recompute the X and R . b. Compute the 3s limits for the chart. On the basis of the 3s limits, is the process mean in control? If not, at what point is it first detected to be out of control? c. On the basis of the Western Electric rules, is the process mean in control? If not, when is it first detected to be out of control? The Western Electric Rules Any one of the following conditions is evidence that a process is out of control Any point plotting outside the 3 s control limits Two out of three consecutive points plotting above the upper 2 s limit, or two out of three consecutive points plotting below the 2 s limit Four out of the five consecutive