Control chart
Also known as: Shewhart chart
Definition
A control chart is a time-series plot of a process metric with statistically derived upper and lower limits, used to detect when a process is no longer behaving as it did historically.
In depth
Common chart types include the I-MR (individuals and moving range) chart for individual measurements, the X̄-R chart for averages and ranges within subgroups, and the p-chart for proportions of defective units.
Control limits are typically placed at ±3 standard deviations from the process mean, calibrated from a stable training window. Western Electric rules add pattern detection beyond single out-of-limits points — runs of 7 on one side of the mean, 2-of-3 beyond 2σ, and so on.
A control chart with a signal is not a defect — it is evidence that something measurable has changed. The follow-up is investigation, not necessarily action.