# In phase i control chart usage that the process was

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In Phase I Control Chart Usage that the process was in control in the past, and the trial control limits are suitable for controlling current or future production.

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In Phase I Control Chart Usage It is highly desirable to have 20 25 samples or subgroups of size n(typically n is between 3 and 5) to compute the trial control limits.
Suppose that one or more of the values of either or R plot out of control when compared to the trial control limits. Clearly, if control limits for current or future production are to be meaningful, they must be based on data from a process that is in control.

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Revision of Control Limits Therefore, when the hypothesis of past control is rejected, it is necessary to revise the trial control limits.
Revision of Control Limits This is done by examining each of the out-of- control points, looking for an assignable cause. If an assignable cause is found, the point is discarded and the trial control limits are recalculated, using only the remaining points. Then these remaining points are reexamined for control.

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Revision of Control Limits (Note that points that were in control initially may now be out of control, because the new trial control limits will generally be tighter than the old ones.)
Revision of Control Limits This process is continued until all points plot in control, at which point the trial control limits are adopted for current use.

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Interpretation of x and R Charts
Common Patterns x and R Charts Cyclic patterns occasionally appear on the control chart. Such a pattern on the chart may result from systematic environmental changes such as temperature, operator fatigue, regular rotation of operators and/or machines, or fluctuation in voltage or pressure or some other variable in the production equipment.

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Common Patterns x and R Charts R charts will sometimes reveal cycles because of maintenance schedules, operator fatigue, or tool wear resulting in excessive variability. In one study in which this author was involved, systematic variability in the fill volume of a metal container was caused by the on off cycle of a compressor in the filling machine.

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Common Patterns x and R Charts A mixture is indicated when the plotted points tend to fall near or slightly outside the control limits, with relatively few points near the center line. A mixture pattern is generated by two (or more) overlapping distributions generating the process output.

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Common Patterns x and R Charts Shift in Process Level These shifts may result from the introduction of new workers; changes in methods, raw materials, or machines; a change in the inspection method or standards; or a change in either the skill, attentiveness, or motivation of the operators.
• Fall '19
• Control Chart, Process capability, W. Edwards Deming, R Charts

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