PHASE 2 SPC METHODS (Variables control chart)
Montgomery p. 204
Done after Phase 1 methods find initial and eliminate assignable causes of special cause variation.
Use methods with more statistical power (EWMA, CUSUM, other) than X-bar and R for faster shift detection without inflating Type 1 error.
Discourage use of sensitizing rules (runs rules) in X-bar and R charts for shift detection of a stable process (Type 1 error).
Tolerance chart (or tier diagram)
Used for ongoing monitoring of a process (outputs and/or inputs) to ensure that any changes in process variation are identified and addressed quickly during ongoing production.
See also: Phase 1 SPC