Statistical process control (SPC) is the application of statistical methods to identify and control the special cause of variation in a process. Control charts, in theory, are used in product and process development to analyze processes. When a process is shown to be in control in both an average and range chart the process can be released for use in production. In practice, however, SPC is often introduced into a production environment where not all processes are in control and fully capable.
When implementing SPC, activities and responsibilities should be clearly established. In the first loop (control loop, see Figure 1) of SPC, an operator will look at subgroups to address any items that are out of control in the process. When the operator is not able to handle the out-of-control variables in the first loop – or if preventive action is required – the second level should be activated (improve loop). At this stage, production management or engineering personnel can take appropriate actions to avoid recurrence of the problem.