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ANOM control limits HELP!

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  • June 19, 2003 at 7:41 pm #32571

    Carl H
    ★ 20 Years ★
    Participant

    I am interested in using an ANOM chart, but want to know how to manually derive the control limits.  I know MINITAB and other packages auto calc this for me, but I want to compare the ANOM limit calcs to the  standard XbarR or S limit calcs.
    Anyone have the limit calculations?
    Thanks
    Carl
     

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    June 19, 2003 at 8:15 pm #87127

    faceman888
    ★ 10 Years ★
    Participant

    It just does a hypothesis test with the H0 being that each mean is = to the grand mean.  I think Ha is not equal (2 sided test) So I guess you would just calculate the decision limits for a t test for the appropriate number of df’s and your selected significance.
     
    If you go to minitab.com go to support>answers database – do a search on it.  If the don’t have an answer in the dbase, log in, go contact us>ask customer support, they will usually get you the answer the same day.

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    June 19, 2003 at 8:55 pm #87128

    Erik L
    ★ 10 Years ★
    Participant

    Carl,
    The control limits for an ANOM analysis are set at:
    Grand Avg +/- (H)*(est. sd(xbar))
    H is specified by three parameters:

    degrees of freedom for estimate of sd(xbar)
    alpha-level
    # of subgroup averages to be compared
    There are tables for the H statistic in Understanding Industrial Experimentation, Wheeler.
    Regards,
    Erik

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    June 20, 2003 at 6:09 pm #87171

    Loehr
    ★ 10 Years ★
    Member

    Hi Carl,
    Erik has the right formula, I only want to mention that a ANOM graph has decision limits, not control limts.  It’s a small point, but it’s important to understand the difference between ANOM and control charts.  Wheeler does a great job of describing this difference.
    I have used ANOM with good results and wish you the best.

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    June 20, 2003 at 7:02 pm #87175

    Carl H
    ★ 20 Years ★
    Participant

    Thanks all for your input.
    I did open the macro for MINITAB Anom and quickly found that the H statistic calculation looks pretty cumbersome…
    I will try to get to the H table values you described.
    One thing I found interesting is that for random generated normal data with 10 pt subgroups, Anom seemed to have slightly tighter decision limits versus XbarR or S  control limits.
    I used ANOM on residuals from a time series regression I did with MONTH as the factor.  It did show significance for one month using ANOM where it did not using control charts of resid. subgrouped by month.
    Thanks
    Carl
     
     
     

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    June 20, 2003 at 7:21 pm #87178

    faceman888
    ★ 10 Years ★
    Participant

    Carl,
    One contributor might be the significance level of the decision limits.  The default in ANOM in mintiab is 95%.  The default for control chart limits is +/- 3 std dev which I think gives a significance around 99.74%.  That coupled witht  the different statistics — H vs some estimate of std dev should account for the difference between the ANOM and control chart limits.

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