Control charts can indicate problems with the process when assignable causes exist in the system. The process with assignable causes is said "out-of-control". In this case, some data points for the control charts will be out of the control limits. In other words, if any measurement or test data is outside the control limits, we will know that something wrong exists in the process. Definitely, it is the time to take corrective actions. Show
A control chart can indicate an out-of-control condition even though no single point plots outside the control limits, if the pattern of the plotted points exhibits non-random or systematic behavior. In many cases, the pattern of the plotted points on control charts will provide useful diagnostic information on the process, and this information can be used to make process modifications that reduce variability. In order to make full use of the information, the problem solving team should have expertise in both statistical principles of the control charts and the process.
a. Cyclic Pattern Figure 2. When cyclic pattern occurs, it is important to determine the cycle interval and time. This cyclic information will usually provide clues as for what may be the causes for the cyclic change.
Figure 3. A shift in process level c. Trend:
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