Question

What is the underlying reason for placing the control limits at 3 standard deviations from the mean?

Answer 1

Solution :-

Underlying reason for placing the control limits at 3 standard deviations from the mean

Control limits on a control chart are commonly drawn at 3 standard deviation from the center line because 3-sigma limits are a good balance point between two types of errors:

Type I or alpha errors occur when a point falls outside the control limits even though no special cause is operating. The result is a witch-hunt for special causes and adjustment of things here and there. The tampering usually distorts a stable process as well as wasting time and energy.

Type II or beta errors occur when we miss a special cause because the chart isn’t sensitive enough to detect it. In this case,will go along unaware that the problem exists and thus unable to root it out.