Control limits are calculated from the process data, typically the centerline plus and minus 3 sigma of the plotted statistic, using 20 to 25 subgroups collected while the process ran stably (a Phase I study). For an X-bar chart with subgroups of 5, the limits sit at the grand mean plus or minus 0.577 times the average range. They describe what the process does and have no connection to what the customer wants.
Don't recalc the control limits off last week's data, the chamber was drifting the whole time. Use the baseline from before the PM.
Drawing spec limits on a control chart in place of control limits is the classic rookie mistake: the chart stops detecting change until product is already bad.
Also heard as
- natural process limits
- 3-sigma limits
Do not mix it up with
- Specification limits: Specification limits come from the design or customer and define acceptable product; control limits come from process data and define normal behavior.
Related terms
UCL
Upper control limit: the top boundary on a control chart, paired with the lower control limit (LCL) below the centerline.
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
Specification limits
Acceptance boundaries for a product characteristic set by design or customer requirements, labeled USL and LSL.
Rational subgroup
Group of measurements chosen so that only routine, short-term variation can occur within it.