What is a Control Chart?
A control chart plots process measurements over time against a center line and statistically derived upper/lower control limits (typically ±3σ). Points inside the limits are normal variation; points outside — or seven consecutive points on one side of the mean (the "rule of seven") — signal the process is out of control and needs investigation.
The exam distinction: control limits describe what the process does (statistics); specification limits describe what the customer wants (requirements). A process can be in control and still out of spec.
Formula
Control limits = mean ± 3 standard deviations
Worked example
A precast-concrete plant charts cylinder strengths daily. All points sit within limits, but nine in a row drift below the mean — the rule of seven fired two days ago. Investigation finds a new aggregate batch. Nothing "failed" yet; the chart caught the change before anything did.