Control charts provide a way of objectively defining a process and variation. They establish measures on a process, improve process analysis and allow process improvements to be based on facts. Note: variation is briefly described here to put control charts in perspective.
The intent of a control chart is to determine if a process is statistically stable and then to monitor the variation of stable process where activities are repetitive. There are two types of variation:
Common or random causes of variation – are inherent in every system over time, and are a part of the natural operation of the system. Resolving common cause problems requires a process change.
· Special causes of variation – are not part of the system all the time. They result from some special circumstance and require changes outside the process for resolution.
Common causes of variation are typically due to many small random sources of variation. The sum of these sources of variation determines the magnitude of the process’s inherent variation due to common causes.
From the sum, the process control limits and current process capability can be determined. Accepted practice uses a width of three standard deviations around the population mean (µ±3δ) to establish the control limits. A process containing only common causes of variation is considered stable, which implies that the variation is predictable within the statistically established control limits.Control Chart