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Estimating Specifications

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  • #54005

    Bill
    Guest

    Is it possible to estimate the range of individual values from two samples? A customer wants us to do specifications but we only have two samples.

    Thanks.

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    #192747

    Chris Seider
    Participant

    Hmmm, it is nice that the customer is willing to make THEIR specifications based on YOUR process variation. Would you assume a normal distribution?

    The confidence interval for standard deviation is so wide with n=2, you can’t be sure of your real process variation so setting control limits or spec limits would seem to be full hardy.

    My 2 cents…or is it 2 observations. :)

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    #192790

    gomezadams
    Member

    Isnt this an oxymoron? Estimating specifications?
    Chris is right on!
    You end up with a process you could throw a football field through.

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    #192791

    Robert Butler
    Participant

    The short answer is yes. You can use the two samples to generate an estimate of the range R = abs(sample 1 – sample 2). Then you can divide this value by 1.128 to get an estimate of the standard deviation and press on from there.

    Unless you have some very compelling reason for doing this – extreme costs with respect to sample generation/testing, a 500 pound gorilla threatening you with severe bodily harm unless you provide an answer, a need for a very quick/rough estimate of how things stand(with the understanding that it is indeed a VERY rough estimate), etc. I would recommend that you not use this to generate spec limits for all of the reasons already cited.

    If you need the particulars with respect to the magic number 1.128 and the above reasoning check Quality Control and Industrial Statistics – Duncan 4th edition pp.140-141

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    #192795

    MBBinWI
    Participant

    Bill – Wow! Specs should be related to the functional need(s) of the design. Specs have nothing to do with part variation (other than comparing variation to range to evaluate capability).

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    #192799

    gomezadams
    Member

    What type of spec is it?
    We may be able to provide more meaningful feedback once we know.
    eg Is it an electrical parametric measurement,mechanical,….?

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