What is SPC? Reading factory control charts
SPC means statistical process control: using production data to monitor variation and investigate changes in a process. Factory control charts can reveal unusual behaviour, but cannot prove that a shipment meets your specification. Cambridge China Bridge helps buyers check factory evidence through its staff in China.
What SPC measures
Statistical process control compares current process behaviour with an established baseline. A control chart plots a measured characteristic or a sample statistic in production order, with a centre line and statistically derived control limits. Useful characteristics might include a moulded part's diameter, a filling weight or a coating thickness.
For a buyer, the useful question is whether the process making your goods behaves consistently. Ask which characteristic the chart monitors and why it matters to your product. A chart for an easy measurement tells you little about a different feature that causes customer complaints.
Read signals, not just a green result
A point outside a control limit is a reason to investigate, rather than automatic proof of defective goods. Patterns within the limits can also suggest a change: a sustained shift, drift or other systematic behaviour deserves attention. Ask the factory which signal rules it uses and what action each signal triggers.
Control limits describe expected process behaviour; specification limits describe what you agreed to buy. A stable process can consistently produce unacceptable parts. Conversely, an unusual process signal can occur while the measured goods still meet specification. Process capability compares a stable process's variation with specification limits; a capability claim needs its supporting data.
Ask for records behind the chart
Request the underlying readings in production order, with dates, batch references, machine or tooling identity, units and the measurement method. Ask how samples were selected and grouped, how the baseline was established, and whether different machines, product versions or material batches were combined. Request explanations for missing readings, excluded results and revised control limits.
Check the measuring equipment, calibration records and whether operators obtain consistent results on the same parts. Ask for the spread chart as well as the average chart where the factory uses grouped measurements: averages can conceal variation. Our guide to checking factory measurements covers the measurement checks.
What a chart cannot prove about your shipment
A chart supports conclusions about the characteristic, process and period represented by its data. It cannot establish that every shipped item meets specification, that unmeasured features are acceptable, or that the cartons contain the goods recorded. Ask the factory to connect the readings to your order and identify any reworked or substituted goods.
Keep the shipment acceptance decision separate. Use the agreed specification, inspection results and any relevant product tests for the actual batch. Our quality inspection guide explains inspection stages; AQL sampling in practice explains sampled acceptance. Neither a stable chart nor a passed sample check guarantees every item.
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Agree what happens when a signal appears
Before production, agree who reviews charts, who investigates signals, how potentially affected goods are identified and held, and what evidence permits release. Ask for the suspected cause, the action taken and subsequent readings showing whether the change worked. Do not accept recalculated limits as the only explanation for a disappearing signal.
If problems recur, connect chart signals with inspection defects and customer returns using batch references. Our guide to supplier quality slipping covers tightening specifications and corrective action. Cambridge China Bridge's staff in China can help buyers request these records and check them alongside the goods.
Frequently asked questions
What does SPC stand for in manufacturing?
SPC stands for statistical process control. It uses process data to monitor variation and identify changes that need investigation.
Does a control chart prove my shipment is good?
No. It describes the monitored process and characteristic. Check that its records cover your order, then assess the shipment against your specification and agreed inspection criteria.
Are control limits the same as tolerances?
No. Control limits reflect expected process behaviour. Tolerances are specification limits for acceptable goods. A process can be stable and still produce parts outside tolerance.
What should I ask a factory for with its SPC chart?
Ask for underlying readings, batch and machine references, the sampling and measurement methods, baseline calculations, signal rules and records of investigations and corrective action.