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industry standards for Cp and Cpk…

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Viewing 19 posts - 1 through 19 (of 19 total)
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  • #51322

    PRPatel
    Participant

    Hello,
    I’m looking to see if there are industry stanrards for Cp.  I know that 1.33 is used for Cpk, but I have not found anything for Cp.
     
    Any thoughts?
    -Pritesh.

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    #177646

    Brian M
    Participant

    Cp tells you if your process is capable, Cpk tell you how centered you are. Most requirements are for Cpk as it is an indication of how much good product you are producing. A great Cp (capable process) is worthless if you are producing outside of the spec limits. A perfectly centered process will have Cp=Cpk.

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    #177650

    marco duarte
    Participant

    I think you could use 1 for CP. This number is acceptable.
     
    Regards!!

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    #177651

    Mikel
    Member

    Wrong, 1 is not acceptable.

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    #177663

    Mike Carnell
    Participant

    Stan,
    Are you sure? The original post was with a Cpk of 1.33 and this guy is ok with a Cp of 1.0. I would really like to see the data for that process.
    Where does this stuff come from?
    Regards

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    #177667

    Van Loon
    Participant

    In the first place its hard to get Cp<Cpk.
    Jan

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    #177668

    Mike Carnell
    Participant

    Jan,
    Really.
    Thank you.

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    #177683

    Ron
    Member

    Many customers are requiring a Cp and Cpk of 1.67 short term and extended requirement of 2.0
    The idea is to have both measurements equal.

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    #177686

    walden
    Participant

    Are you saying customers expect higher long term capability than short term?

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    #177691

    Mikel
    Member

    That would be a pretty dumb thing to say – don’t you think?

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    #177710

    Mario Perez-Wilson
    Participant

    Ron, your statement “The idea is to have both measurements [Cp=Cpk] equal” is well taken, however, it is a good general statement, particularly, when you are focusing only on a single domain, such as quality by reducing defects.
    It may NOT be a good generalization when we are characterizing (concurrently) in more that just one domain. Quality is one domain, but there are other domains, such as speed, cost, etc.
    Let’s state some underlying assumptions, so the readers don’t derail.A1 – The Cp can never be smaller than the Cpk.
    A2 – The Cp and Cpk cannot be negative.
    A3 – Knowledge exists on how to control the average and the variance.
    A4 – Dealing with variable data under the Ratio scale of measurement.
    A5 – Maximizing a domain does not imply maximizing its value.
    A6 – A domain may have multiple responses.
    In the quality-domain, having the Cp equal to the Cpk, implies that the average is centered in the middle of the specification limits, and this in turn minimizes the probability to produce outside the specification limits (below the lower spec. limit and above the upper spec. limit). However, when now, we introduce a second domain, the general idea of having the Cp=Cpk, may not be the way of maximizing for both domains. When dealing with two domains, quality and cost, a better choice may be to maximize the Cp and not maximize the Cpk. Maximizing Cp may work best for quality and cost. However, maximizing the Cpk may be best for quality but not cost. As a matter of fact, maximizing the Cpk may be detrimental in the cost-domain. A good example is Gold Plating, particularly at a spot price of 750 USD per oz. In a gold plating process, I may want to maximize the Cp, but I may want to have a Cpk that meets the standard but with an average as close to the lower specification limit as I can get. This in turn guarantees that I am maximizing under the quality-domain (less probability of producing beyond the specification limits) and maximizing under the cost-domain (by providing the minimum gold or less cost, thus more earnings), and still meeting customer satisfaction.
    When we characterize processes under the MPCpS Methodology we are working on maximizing five domains.
    So, the statement of Cp=Cpk is good, but it may depend on the process and under how many domains I may be characterizing that process.

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    #177711

    Van Loon
    Participant

    A2 – Wrong – Cpk can very well be negative.
    Jan

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    #177712

    Gary Cone
    Participant

    Jan,Take the time to think about what Mario is saying. This is an
    ASSUMPTION. He is saying we will not choose to run a process outside
    the spec.Mario is sharing something well beyond what is normally seen or for
    that matter what is normally taught. Take the time to understand.

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    #177718

    Anonymous
    Guest

    Gary,Did a couple of your posts disappear?Cheers,
    Andy

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    #177719

    Gary Cone
    Participant

    Andy,Yes.I am too political these days and criticized a big advertiser.No problem, saw your message on Lasik and am doing some
    digging as I am considering.If you have time go to http://blog.gpsqtc.com and comment again.
    About 100 hits per day there, so a possibility for an unedited
    discussion.I value you opinion.Weigh in on Mario’s comments as well. Also an ex-Motorolan and a
    good guy.

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    #177722

    Anonymous
    Guest

    Gary,If someone took your post down because you criticized an advertiser that is a great shame because I think rational argument and the ability to reason is a very important part of six sigma implementation.As there is an eye in isixsigma, I hope the powers that be won’t mind me providing you with this link as your blog has a password:http://www.usaeyes.org/lasik/library/fda-lasik-quality-study.htmYes, I’ll read Mario’s post over the next few days.Do you know Dr. Pyle? I’m meeting him and his family in London next week.Cheers,
    Andy

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    #177723

    Anonymous
    Guest

    Mario,I’m not sure about your use of the term ‘domain’ and your claim Cpk can’t ever be negative, but your other comment is an important contribution:Another example of your multivariate approach not only applies to different variates, but also the same variate, such as a flow controller, where parts identically machined can cause defects in the centre of the specification due to stiction.Cheers,
    Andy

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    #177729

    Gary Cone
    Participant

    I don’t know Dr. Pyle – who is he?

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    #177730

    Anonymous
    Guest

    Hi Gary,Dr. Pyle was formerly director of IP (Motorola SPS) in Austin. He is now an IP attorney and expert witness.Cheers,
    Andy

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