SPC (statistical process control) is a method for monitoring a production process with statistics, so problems are spotted while the process is running instead of after defective products have been made. Measurements such as dimensions, weights or temperatures are plotted on control charts, and statistical rules show when a process drifts away from its normal behaviour.
How SPC Works
Every process varies a little. SPC distinguishes between common-cause variation, the normal noise of a stable process, and special-cause variation, a signal that something has changed: a worn tool, a new batch of material, a machine setting. Measurements are taken at regular intervals and plotted on a control chart with a centre line and upper and lower control limits, usually three standard deviations from the mean. A point outside the limits, or a pattern such as a run of points on one side of the centre line, is a signal to investigate before the process produces scrap.
Common Control Charts
- X-bar and R charts for the average and range of small samples of measured values.
- Individuals charts for single measurements, such as one value per batch.
- p and np charts for the share or number of defective items.
- c and u charts for the number of defects per unit.
Process Capability
Where control charts show whether a process is stable, capability indices such as Cp and Cpk show whether a stable process fits within the specification limits the customer requires. A process can be perfectly stable and still produce too many parts out of tolerance.
SPC in Software
Paper charts are slow and easy to skip. SPC software collects measurements from gauges, machines or operators, updates control charts in real time, raises alarms when a rule is violated and links each signal to the corrective action that followed. See quality control software and machine data processing.
Frequently Asked Questions
What is the difference between control limits and specification limits?
Control limits are calculated from the process itself and show what is normal. Specification limits are set by the customer or design and show what is acceptable. Both are needed.
Is SPC the same as quality control?
SPC is one part of quality control: it monitors the process. Quality control also includes inspections, handling non-conformities and corrective actions.
Who invented SPC?
Walter Shewhart developed the control chart in the 1920s at Bell Laboratories; W. Edwards Deming later spread SPC widely in industry.