Statistical Process Control (SPC)
Definition
A method of monitoring a manufacturing process in real time using control charts that distinguish normal process variation (common cause) from signals of an out-of-control process (special cause). SPC records become evidence of process state at the time of manufacture.
- Chart type
- Shewhart control charts (X-bar, R, p, c)
- Limits
- Typically set at +/- 3 standard deviations from the mean
- Signal type
- Special-cause variation, distinct from normal common-cause noise
- Forensic use
- Records tie a defect to a specific out-of-control window
Common questions
How does SPC data help prove when a defect entered production?+
Control chart records timestamp when a process parameter left its limits, so investigators can match the out-of-control window to specific production lots or serial number ranges and rule in or out affected batches.
Does an in-control SPC chart mean no defective units were made?+
No. Common-cause variation still produces a background rate of defects even while a process stays within control limits, so an in-control chart shows the process was stable, not that every unit was flawless.
Related terms
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- ISO 9001
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- Manufacturing Defect
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- Race Condition (Software)
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