Manufacturing and operations · Level 3 to 5. Quality inspectors, line leaders, process engineers and quality managers
In line quality and statistical process control
Control charts, capability, and the difference between a process that is stable and one that is good.
- 2modules
- 4.5CPD hours
- 240guided minutes
- Freemodule 1
About this course
Statistical process control is taught almost everywhere as a charting technique and understood almost nowhere as a decision rule. The result is plants covered in control charts that nobody acts on, and adjustments made in response to variation that was never a signal.
This course teaches the two kinds of variation and why the distinction decides who should respond; how a control chart is read, including the run and trend rules that most training omits; the single most damaging error in the subject, which is treating control limits as specification limits; and process capability, where Cp and Cpk together tell you whether a process is too variable, off centre, or both.
The second half is what happens when the chart says stop: quality gates, holds, disposition, segregation of duties, and the traceability that turns a recall from a catastrophe into an afternoon.
What you will be able to do
- Distinguish common cause from special cause variation and route each correctly
- Read a control chart and apply the beyond limits, run and trend rules
- Explain why control limits and specification limits must never be confused
- Recognise tampering and explain how over adjustment increases variation
- Calculate and interpret Cp and Cpk, including what a gap between them means
- Apply a quality gate and disposition a held batch correctly
- Explain the segregation of duties around scrap and release decisions
Modules
Assessment and certificate
Knowledge check after each module and a final assessment at 80 per cent, with unlimited attempts.
A free digital certificate showing 4.5 CPD hours with a verification code. It evidences knowledge of the method. Authority to release, rework or scrap product in your own organisation is granted by that organisation, not by a certificate.
Questions
What is the difference between control limits and specification limits?
Control limits come from the process and describe what it does. Specification limits come from the customer or the designer and describe what is acceptable. They are unrelated. Drawing specification limits on a control chart is the commonest error in the subject, because it invites the team to adjust a stable process every time a point strays towards a tolerance it was never meant to be compared against.
Our process is in control but we still make defects. How?
In control means stable and predictable, not good. A process can be perfectly stable and consistently produce output outside specification. Stability tells you the process is behaving; capability tells you whether what it produces is acceptable. You need both, and they are answered by different measures.
Why is adjusting after every out of specification part a bad idea?
Because most individual results reflect common cause variation, which is the process behaving normally. Adjusting in response to it adds your correction on top of the natural swing and makes the output more variable, not less. This is called tampering, and it reliably makes things worse while feeling responsible.
Who should be allowed to scrap a batch?
Not the person under pressure to hit the output number, which is the whole argument for segregation of duties. A common arrangement is that quality may release or rework at any quantity, while scrapping above a defined value requires a second, more senior authorisation. The threshold matters less than the principle that the decision and the pressure sit with different people.