Control Chart
A graph that shows whether a process is stable and within acceptable control limits over time.
A control chart is a statistical quality tool used during the Control Quality process to monitor whether a process is performing within established limits over time. It displays a center line (the mean), an upper control limit (UCL), and a lower control limit (LCL), typically set at plus or minus three standard deviations from the mean. Data points within those limits and showing no non-random patterns indicate a process in statistical control.
The critical exam distinction is between common cause variation — natural, random fluctuation inherent to any process — and special cause variation, which signals an assignable, abnormal factor that must be investigated. Points outside the control limits always indicate special cause variation, but non-random patterns within the limits can too, which is where the rule of seven applies. Control limits are NOT the same as specification limits; specification limits reflect customer requirements and are set externally, while control limits are derived from the process data itself.
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