When to use it
Use it for repeated, comparable observations collected in time order.
When to be cautious
Control limits describe process behaviour; they are not customer specification limits. Chart choice and subgrouping matter.
Apply it
- Define the measurement and a suitable sampling or subgrouping method.
- Choose and establish the appropriate chart from a stable reference period.
- Investigate signals and verify process changes before recomputing limits.
An example in practice
Repeated turnaround-time observations reveal a shift after a workflow change. Investigate the change rather than explaining every fluctuation separately.
Source & credit
Walter A. Shewhart developed statistical control charts. NIST is a later technical reference, not their inventor.
This explanation and example are independently written. The source is a reading reference; linking it does not grant permission to reuse its text, figures or assessments.