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 data from the last 30 samples are presented in Table E12 on page 788. The means are X̅ = 150.075, R̅ = 6.97, and s̅ = 3.082.

a. Compute the 3σ 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 X̅ and R̅.

b. Compute the 3σ limits for the X̅ chart. On the basis of the 3σ 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?