Control chart x and s for are maintained on a process. After 25 preliminary subgroups each of size 3 are evaluated, you have the following data:
(a) Set up control charts using these data.
(b) Assume that the process exhibits statistical control. Estimate the process mean and standard deviation.
(c) Suppose that the quality characteristic is normally distributed with specifications at 2.25 + – 4. Estimate the fraction nonconforming produced by this process.
(d) How much reduction in process variability would be required to make this a Six Sigma process?
The Motorola Six Sigma concept is to reduce the variability in the process so that the specification limits are at least six standard deviations from the mean (text p. 29). If the process mean was centered at the specification midpoint, this would require a standard deviation of