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Control Chart: What It Shows, How to Read It, and How It Differs from a Run Chart
A control chart plots a process measurement over time against calculated upper and lower control limits, so you can tell normal variation from a real problem. It looks like a run chart with lines added, but those lines change everything: a run chart shows the trend, a control chart tells you when to act.
Why is a control chart important?
A control chart matters because it tells you the difference between normal process variation and a real problem, so you act only when action is genuinely needed. That prevents both missed defects and needless tampering, which is itself a common cause of added variation. As the core tool of statistical process control, it turns raw measurements into a clear signal of when to intervene.
How do you read a control chart?
The chart separates two kinds of variation, and only one needs action:
- A center line, the process average, with an upper and lower control limit either side.
- Points inside the limits and randomly scattered mean common-cause variation, the normal noise.
- A point outside a limit, or a non-random pattern, signals special-cause variation, something changed.
- You act on special cause and leave common cause alone, because tampering with noise makes it worse.
How does a control chart differ from a run chart?
The limits. A run chart plots the same data over time but has no control limits, so it shows movement without telling you whether it matters. A control chart adds statistically calculated limits, which is what turns it into a decision tool and the backbone of statistical process control. One shows the trend; the other flags when to intervene.
Control chart vs run chart at a glance
The same data, one with limits.
| Aspect | Control Chart | Run Chart |
|---|---|---|
| Has control limits | Yes | No |
| Tells you | When to act | That something moved |
| Separates | Common vs special cause | Nothing; just the trend |
| Use | Process control (SPC) | A quick visual |
Related Terms
- Quality Control – where the chart is used.
- DPMO – a defect metric alongside SPC.
- Root Cause Analysis – what a special-cause signal triggers.
- Computer
- First Article Inspection (FAI)
Further Reading
- QMS-Software – digital quality management
