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Cp and Cpk value..

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  • #41519

    hanna
    Participant

    Hye..
    I need someone to explain to me about the Cp and Cpk value.
    Can it take the negative value,eg -1.23?
    Do please help me on this coz it urgent..
    Thank you for helping me.

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    #130270

    Kumar
    Participant

    give me ur id, i can get u some materials on this…

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    #130271

    Kumar
    Participant

    give me ur id, i can get u some materials on this…

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    #130288

    hanna
    Participant

    sorry..i dont know what is my id…
    is there any other way for me to reach you and the materials?
    sorry again..
     

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    #130291

    rajakkumar
    Participant

    Cp and Cpk do not have any units and therefore dont have any positive or negative sign.Please check your working.
    regards,
    K.rajakkumar

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    #130292

    hanna
    Participant

    If  I want to monitor daily process performance, can I used this Cp and Cpk value to make conclusion that for a certain day my process is met specification?
    2ndly, may I know which is more useful: using control chart alone or control chart with Cp and Cpk value?  I already got confuse to use the SPC tools..
    Lastly, do you know any information regarding SPC format? (make a table using XLs to monitor process performance)

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    #130294

    McD
    Participant

    Cp is simply the distance between the spec limits (which must be positive) divided by 6 times the standard deviation (which must be positive).  Cp can therefore never be negative.
    Generally, Cpk is more useful.  Cp assumes that your process is centered between the spec limits, which is not something you can generally guarantee.  You can have a very good Cp and be producing all your product out of spec.  Cpk is the distance to the closest spec limit divided by 3 times the standard deviation.
    I guess if your process mean was out of spec. then you could come up with a negative Cpk.  This would be such a serious problem, though, that you should be fighting fires rather than worrying about your process capability.  What is says is that most of what you are producing is out of specification.  You would already know this, however, without calculating Cpk.
    If, for example, your specs were 1 to 3, and your process mean was 4, then you would have a negative Cpk.  But since you know your process mean is outside your spec range, you would already know you have a problem (unless you happened to be asleep).
    But as long as your process mean is between your specification limits, then Cpk must be positive.
    –McD
     

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    #130295

    Kumar
    Participant

    my e-mail id is [email protected]
    you can reach me in this…

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    #130296

    hanna
    Participant

    So I need to focus on the causes of the problem. It means that there’s no point to know Cp and Cpk if the process is out of control rite? And the rite solution here is I need to monitor the process whether it is out of control by using control chart. And here I got 1 more question, which one of this chart gives me a clear picture of my overall process performance?: Xbar-R chart or Xbar-S chart?

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    #130299

    sumpat
    Member

    any one have some tools for cal CP and CPK ?

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    #130300

    hanna
    Participant

    what kind of tools u want? the simple method i know is using xl.

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    #130302

    McD
    Participant

    If your calculation is correct, and you should check it, then what it is saying is that the process mean is outside the spec limits.  But if it is, you don’t need Cp or Cpk or even a control chart to tell you that.  You have a serious problem that needs to be addressed.  And it is an obvious problem!
    If your process mean is within the spec limits, and that is pretty easy to test, and your Cpk is negative, then your calculation for Cpk is wrong.  It is fairly easy in Excel to get the sign wrong!
    So look to your process mean.  If it is outside the specification limits, you have an obvious problem and you should forget about Cpk and control charts and all that nonsense until you at least get the process onto the right planet.
    When quality guys talk about a process being “in control”, they are usually referring to the Shewhart tests for a control chart.  But the assumption here is that generally, the process mean is somewhere near where it needs to be.   If your specification is 1 to 3, and you are producing product at 10, you don’t need a control chart to tell you that there is a problem.
    However, if your specs are  to 3, and your process is running around 2.5, then your Cpk can’t be negative.  A negative Cpk indicates a problem in your spreadsheet.
    Remember that for Cpk, you need the distance to the nearest spec limit.  So if X is the process mean, and LL and UL are the lower and upper spec limits, you calculate X-LL and UL-X and take the smaller of the two.  If X is between LL and UL, this result will always be positive.  Pretty easy in Excel to get one or both of those swapped.
    –McD
     

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    #130328

    Fact man
    Participant

    A negative Cpk value means the average is outside of the spec range. Try adjusting the average to teh center of the spec and you will get a positive Cpk value.

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    #148587

    kuldeep mujoo
    Participant

    dear sir,
    please give me information about cpk values
     
    with regards
    kuldeep

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    #161218

    Quainoo
    Member

    Will Cpk value higher then Cp value?

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