In quality-control projects, engineers use charts where item values are plotted and compared with 3-standard-deviation bounds above and below the mean for the process.
When items are found to fall outside the bounds, they are considered nonconforming, and the process is stopped when "too many" items are out of bounds.
Assuming a normal distribution of item values, what percentage of values would you expect to be out of bounds when the process is in control? Accordingly, how would you define "too many"? What do you think is the rationale for this practice?