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    Statistical process control involving R-chart & x-bar chart

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    Need help with problems on statistical process control involving R-chart & x-bar chart.


    What happened at different times during this shift?
    Why did management change the UCL in the R chart just after 10 a.m.?
    Why did it change the LCL in the R chart after 2 p.m.?
    Why did it change the control limits on x-bar chart at 3 p.m.?

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    Solution Preview

    Statistical process control

    In the R chart exactly at 9am and 10am there was one defect each that happened close to the UCL and inside the UCL. So after 10am the UCL was tightened and the third defect that occurred at 11am fell outside the UCL! The net result was that the defects inside the UCL were reduced.

    Before 2 pm at 12 noon there occurred a defect close to the LCL and again at 1 pm there occurred another defect very close to the lower control limit. So, at 2 pm the LCL was tightened to eliminate any defects from falling inside the LCL. The action was successful as the two defects that occurred at 3pm and 4pm fell outside the LCL.

    In the X chart, from 9 pm to 3 pm there occurred 7 defects of which 5 were either on control limits or inside the control limits. The two that were outside the control limits narrowly missed the control limits. At 2 pm such a defect happened that narrowly ...

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