Action and warning lines

Use action and warning lines in quality assurance sampling applications.

Control charts used for sample mean, median or range is expected.
For example, in a manufacturing process to test that certain measurements are within allowable limits.
Understand that almost all means, medians or ranges fall inside the action lines (action limits), and only 1 in 20 fall outside the warning lines (warning limits).
Know that warning lines are set at ±2 standard deviations of the sample mean from the expected value, and action lines are set at ±3 standard deviations of the sample mean from the expected
value.
Know the action to be taken if a sample value falls outside each type of limit.