Control limits vs specification limits
Short answer
Specification limits (LSL, USL) are the voice of the customer: they come from the drawing and decide whether a single part is good or bad. Control limits (LCL, UCL) are the voice of the process: they are calculated from the process data, usually ±3 sigma of the plotted statistic, and show whether the process is stable. The two are compared only through capability indices such as Cpk; specification limits never belong on a control chart of averages.
Side by side
| Aspect | Control limits (LCL, UCL) | Specification limits (LSL, USL) |
|---|---|---|
| Source | Calculated from process data | Drawing, customer requirement |
| Purpose | Detect special causes, judge stability | Judge conformity of each part |
| Applies to | The plotted statistic (average, range, individual, p) | Individual parts |
| Changes when | The process changes and limits are recalculated | The design or requirement changes |
| Point outside means | Process changed, investigate | Part is nonconforming |
Why mixing them is dangerous
Averages vary much less than individual parts. If specification limits are drawn on an X̄ chart, all averages can sit comfortably inside while many individual parts are outside the tolerance. In the other direction, reacting to the specification instead of the control limits makes operators adjust a stable process, which only increases variation.