Every process varies. Parts are never exactly the same size, deliveries never take exactly the same time, and calls never last exactly the same number of minutes. Statistical process control, or SPC, is a practical way to tell normal variation from a real problem, so teams act at the right time and leave stable processes alone.
Common cause and special cause
SPC rests on one simple idea. Some variation is built into the process, called common cause. Some comes from a specific event, such as a worn tool, a new supplier or an untrained operator, called special cause. Reacting to common cause as if it were special wastes effort and often makes things worse. Missing a special cause lets defects through.
Control charts
A control chart plots a measurement over time with a centre line and upper and lower control limits calculated from the process itself. Points outside the limits, or unusual patterns such as long runs on one side, signal a special cause worth investigating.
Capability
Control tells you whether a process is stable. Capability tells you whether a stable process can meet the specification. Measures such as Cp and Cpk compare the spread of the process to the tolerance the customer requires. Many customers ask suppliers to demonstrate a minimum capability on key characteristics.
Trust your measurements first
If the gauge or measurement method is unreliable, the chart is meaningless. A measurement system analysis, often called gauge R&R, checks how much of the observed variation comes from the measurement itself.
What leaders should ask for
- Control charts on the few characteristics that matter most to customers
- Clear reaction plans for operators when a signal appears
- Regular capability reporting for key processes
- Validated measurement systems
- A routine review of charts by supervisors and managers
Beyond manufacturing
The same methods work for service operations, logistics, finance processes and call centres: anywhere work is repeated and results can be measured.
Used consistently, SPC reduces scrap, rework and complaints, and gives leaders an early warning instead of an unpleasant surprise.
Need help with this? Talk to Creative Contact Consulting Group about your situation.