In short: Statistical process control uses control charts to monitor a process over time and distinguish common-cause variation from special causes. Combined with capability indices such as Cp and Cpk, SPC shows whether a process is stable and able to meet specifications.
What is a control chart?
A control chart plots process data over time with a centre line and upper and lower control limits calculated from the process itself.
Which chart should I use?
- X̄–R or X̄–S for subgrouped variable data
- I–MR for individual measurements
- p and np for proportions of defective items
- c and u for counts of defects
What is process capability?
Capability compares process spread with specification limits. Cp measures potential capability; Cpk also accounts for how well the process is centred.
Key takeaways
- SPC separates common-cause from special-cause variation.
- Control limits come from the process, not specifications.
- Only assess capability once a process is stable.
Frequently asked questions
What is a special cause?
A source of variation that is not part of the normal process, signalled by points outside limits or non-random patterns.
What is a good Cpk value?
Many industries use 1.33 as a minimum, with higher values for critical characteristics.
Can SPC be used in services?
Yes, for example for waiting times, error rates and processing times.