9. Control charts for X and S are maintained on resistors (in ohms). The subgroup size is 4. The values of X and S are computed for each subgroup. After 20 subgroups, X i = 8,620 and



Si = 450.

(a) Compute the values of the limits for the X and S charts.

(b) Estimate the value of σ on the assumption that the process is in statistical control.

(c) If the specification limits are 430 ± 30, what conclusions can you draw regarding the ability of the process to produce items within these specifications?

(d) If μ is increased by 60, what is the probability of a subgroup average falling outside the control limits?