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Help Needed with Control Chart Constants

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  • #52786

    Travis
    Member

    I would like to locate a table of control chart constants (A2, d2, c4,… that shows values for greater than 25 observations.  If you have a copy you wouldn’t mind sharing or can tell me where I can find a copy, it would be greatly appreciated.
    Thanks

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    #186134

    Darth
    Participant

    And in what circumstances would you want to be selecting rational subgroups of greater than 25? Not sure your sampling scheme makes sense.

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    #186747

    Steve Wise
    Member

    As far as I know, there are no d2 factors for n>25. Also, to further Darth’s comment, why would one use a Range statistic on large subgroups? Range only uses two values in the subgroup whereas standard deviation uses all the values.

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    #186750

    Helper
    Participant

    Truth is, nobody on this site can help you with your question.  their knowledge of stats do not go that deep.

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    #186752

    Darth
    Participant

    Obviously, your name is a misnomer….it should be Unhelper since you didn’t contribute anything to the thread but made an inane comment instead.

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    #186755

    Lee
    Participant

    I can help for d2, and know who might be able to help with other constants for large sample sizes.  E-mail me directly at Eugene_Lanning at Cargill.dot com

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    #186760

    Lee
    Participant

    Two comments:

    1.  For values, see https://www.isixsigma.com/tools-templates/control-charts/control-chart-table-of-constants/

    The second one is the source I used to get d2 values for large samples.    What I’m suggesting is that you contact him, as I did, so you get a more direct contact with the person that has not only has an Excel program to get the numbers, but also has copies of background papers that explain the d2 value.

    2.  The request for d2 values for large samples is cropping up more often.  We need to understand why that is — just like looking at other seemingly unusual requests.   My interest was more academic in nature, so that does not count for much.  However, I understand that with the advent of mass production that automated data collection has made larger sample sizes have very low costs.  Is that what is happening in each case?   Posters of the requests for d2 values for n>25 please respond, indeed why not have persons that use d2 values for n>10?

    Technology may be driving us towards re-thinking what is a typical sample size.

    Have a statistically good day.

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    #186779

    Helper
    Participant

    darth,
    That is a compliment coming from you………

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    #186780

    StuW
    Member

    The classic text “Statistical Process Control” 4th edition 1972, by Eugene Grant and Donald Leavenworth has a table in the Appendix, table B, which has d2 and c2 values in increments of 5 between 25 and 100.   The c2 values convert from sigma-bar to an estimate of sigma using “n” in the denominator, not “n-1”.  You can easily interpolate to get desired values in between those groupings of five.
    I would also be interested in hearing about any applications of the use of Range for control chart applications, with sample sizes greater than ten, or so.  
    There is a fairly harsh penalty to pay by choosing to use a Range for samples above this size.   It is much more efficient, statistically, to use the standard deviation from calculated samples, as the sample size increases.   Since I would surmise the sample averages are being computed in an automated fashion, it really takes no more effort to program the use of the standard deviation than the range for these applications.
     

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    #186782

    Lee
    Participant

    Personal suspicion, based on no facts at all, is that when the automated systems were originally installed/created that the sample size was small.  But as computer speeds increase the sample size was increased but the programming was not updated to do the Std Dev method — possibly because some applications the still have small sample sizes.  Another thought is that it was easier to tell the programmer how to use the range and d2 than it is to explain the Std. dev. calculations, so the range is still used.    Like I said, those thoughts are based on zero facts. 
    We need to hear more from those that are using the tables as to why they need d2 for large values.

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