Control Limits and Specification Limits

Control limits and specification limits answer different questions. Control limits ask whether the process changed; specification limits ask whether output satisfies requirements.^[1]

Prerequisites

Prerequisites: control chart basics.

Definition

Control limits are statistical boundaries computed from the process data or from an approved standard process parameter. Specification limits are external requirements such as lower specification limit (LSL), upper specification limit (USL), target value, tolerance, or acceptance criterion.

Key Distinction

Limit type	Source	Used for	Should change when
Control limits	Process behavior	Detecting special causes	The stable process changes or Phase I analysis is revised
Specification limits	Customer, design, regulation, engineering	Judging conformance and capability	Requirements change

Worked Example

A stable process has $\bar{x} = 10.00$ , $\hat{σ} = 0.10$ , so approximate individuals control limits are $9.70$ and $10.30$ . The drawing specification is $9.90$ to $10.10$ . The process can be statistically stable while producing too many parts outside specification; stability does not imply capability.

Common Mistakes

Drawing specification limits on a control chart and calling them UCL/LCL.
Widening control limits to reduce alarms.
Narrowing control limits to match customer tolerance.
Running capability studies before stability is established.

Connections

Related note	Use
Control charts	Where control limits are used
Process capability	Where specification limits are used
Common-Cause and Special-Cause Variation	Signal interpretation
Statistical Process Control	Main hub

References

NIST/SEMATECH, e-Handbook of Statistical Methods, "What are Control Charts?" and "What is Process Capability?", https://www.itl.nist.gov/div898/handbook/pmc/section3/pmc31.htm ↩︎