<P> If the process is in control (and the process statistic is normal), 99.7300% of all the points will fall between the control limits . Any observations outside the limits, or systematic patterns within, suggest the introduction of a new (and likely unanticipated) source of variation, known as a special - cause variation . Since increased variation means increased quality costs, a control chart "signaling" the presence of a special - cause requires immediate investigation . </P> <P> This makes the control limits very important decision aids . The control limits provide information about the process behavior and have no intrinsic relationship to any specification targets or engineering tolerance . In practice, the process mean (and hence the centre line) may not coincide with the specified value (or target) of the quality characteristic because the process design simply cannot deliver the process characteristic at the desired level . </P> <P> Control charts limit specification limits or targets because of the tendency of those involved with the process (e.g., machine operators) to focus on performing to specification when in fact the least - cost course of action is to keep process variation as low as possible . Attempting to make a process whose natural centre is not the same as the target perform to target specification increases process variability and increases costs significantly and is the cause of much inefficiency in operations . Process capability studies do examine the relationship between the natural process limits (the control limits) and specifications, however . </P> <P> The purpose of control charts is to allow simple detection of events that are indicative of actual process change . This simple decision can be difficult where the process characteristic is continuously varying; the control chart provides statistically objective criteria of change . When change is detected and considered good its cause should be identified and possibly become the new way of working, where the change is bad then its cause should be identified and eliminated . </P>

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