Control limits

voice of the process

UCL / LCL = centre line ± 3σ of the plotted statistic

Control limits are calculated from the process data and show the range in which the plotted statistic varies when only common causes act. A point outside the limits signals a special cause.

They are not the same as specification limits: a process can be in control and still produce parts out of tolerance, or be out of control while all parts are good.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Specification limits (LSL, USL): voice of the customer
• Nelson rules: tests for special causes on control charts
• Special cause: assignable cause of variation