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statistical quality control

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A new process is started, and the sum of the sample standard deviations for 25 subgroups of size 4 is 750. If the specifications are 700 +- 80, what is the process capability index?

What I know:
D2=2.059
USL = 780; LSL = 620; Tolerance = 160

  • statistical quality control - ,

    Cp=USL-LSL/6 sigma note
    = 160/6sigma note
    sigma note=s(bar)/c4 ; c from the table for sybgroub size 4
    c4= 0.9213
    s(bar)=750/25=30
    sigmanote=30/0.9213=32.56
    6 simgma note=6* 32.56=195.37
    so
    cp=160/195.37=0.818
    cp<1 , the process in state I

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