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Minitab control charts: test 2 question

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  • #49876

    Rob Thompson
    Member

    When plotting control charts in Minitab, Test 2 = 9 points in a row on same side of center line.If these points are BELOW the centre line, then it potentially indicates a process improvement.If these points are ABOVE the centre line, then it potentially indicates a process improvement.My question is why?

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    #171262

    Rob Thompson
    Member

    Sorry – ignore the above, my message should have read:When plotting control charts in Minitab, Test 2 = 9 points in a row on same side of center line.If these points are BELOW the centre line, then it potentially indicates a process improvement.If these points are ABOVE the centre line, then it potentially indicates a process DEGRADATION.My question is why?

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    #171275

    DaveS
    Participant

    Rob,
    “When plotting control charts in Minitab, Test 2 = 9 points in a row on same side of center line.”
     This is the only part of your post that is strictly true.
    The presence of this pattern suggests that there is sufficient evidence of a special cause to warrant investigation. That is; it is economic when the signal appears to try and find out why. That is the meaning of all the tests.
    So, if you can fnd the special cause and institute it into the process, than an improvement might be possible.
    The part about above the center line and below is situational. What if above the center line is in the direction of defective parts? Larger is not always better.
    The presence of any signal on test 2 will indicate a process improvement possibility. If in the direction of goodness, institutionalize it in the process. If it is in the direction of badness, find a way to prevent it ever happening again.

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    #171277

    Heartland
    Participant

    In a control chart “up” or “down” are both bad in most cases. (yes, I know some things 0 is best and lower would be better, and others most is best – example for low might be time to fill an order, or most might be good product produced per hour).  Even in those limited cases you need to be careful.  For example: if you decrease time to fill an order at a pick/pack operation without looking at the entire system you could cause stock outs because your KANBAN was established at the old speed (next step adjust the KANBAN).
     
    The center, or target, is named target for a reason.  9 in a row indicate something has changed and should be investigated.  If the 9 in a row ALSO show less variation then there is a possibility of a process improvement (if you can re-center to the target.).

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    #171280

    Heebeegeebee BB
    Participant

    all it indicates is a fundamental shift of your process.   It’s a flag that something changed.   It’s your cue to investigate.

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