The statistical boundaries on control charts that signal when a process may be shifting out of control.
Lower Control Limits (LCL) and Upper Control Limits (UCL) mark the expected range of variation for a stable process. Data points outside the limits prompt investigation before defects proliferate.
Calculating control limits
Control limits depend on the chart type, subgroup size and variation estimate. Three-sigma limits apply to the statistic plotted, not automatically to individual product measurements. Specification limits instead come from product requirements.
Interpreting signals
Points outside the limits and specified patterns inside them can signal instability. Investigate using the chosen chart rules. A stable process can still fail product specifications; using a chart with Cpk analysis does not itself make the process capable.
Key takeaways
- Control limits separate common cause from special cause variation.
- Signals outside the limits warrant immediate investigation.