Slide 1
Put measurements in time order
A control chart tracks a process over time against limits based on its expected variation. It helps identify behaviour that deserves investigation.
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A histogram can show the overall spread while hiding a shift that happened midway through production. Keeping the time sequence preserves evidence about when the process changed.
Slide 2
Control limits describe behaviour
Control limits are calculated from a process and a chosen statistical method. They are different from specification limits set by a customer or design requirement.
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A stable process can consistently make unacceptable parts. Conversely, individual parts may meet specifications while the process shows an unusual shift that needs attention.
Slide 3
A signal is a reason to investigate
A point beyond a control limit can indicate a change beyond ordinary variation. It does not automatically identify the physical cause of that change.
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A measurement error, tool wear or changed material could produce a signal. Checking the process and measurement system is necessary before deciding which adjustment, if any, is justified.
Slide 4
Patterns can matter within the limits
A sustained run or trend can contain information even when individual points remain within limits. A monitoring plan can use appropriate pattern rules.
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Adding many rules increases the chance of an alarm from ordinary variation. Detection sensitivity must be balanced with false alarms, rather than assuming more rules always improve control.
Slide 5
Do not chase every fluctuation
Repeatedly adjusting a stable process in response to routine noise can increase variability. Control charts help distinguish a reasoned intervention from unnecessary tampering.
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After a real process change, a new baseline may be appropriate once its behaviour is understood. Continually redrawing limits around each awkward observation would erase the chart’s ability to detect change.
Sources
- What Are Control Charts?
NIST Engineering Statistics Handbook
- What Are Variables Control Charts?
NIST Engineering Statistics Handbook