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LSL = 0 and 0 is target value

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  • May 4, 2007 at 3:44 pm #46896

    Dona
    ★ 10 Years ★
    Participant

    I am doing a balancing process where the lowest value I can have is 0 mg and this is the best part possible. My USL = 20mg.
    My target is 0 mg so should I set mt LSL to -20 even though this is an impossible value to achieve?

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    May 4, 2007 at 4:11 pm #155719

    Vidyadhar
    ★ 10 Years ★
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    Its a little difficult to understand as to why would your target value be 0 if you have an USL of 20 mg & LSL of 0 mg.

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    May 4, 2007 at 4:27 pm #155722

    walden
    ★ 10 Years ★
    Participant

    Your LSL is not 0, a value of 0 is a natural boundary. You have a one sided spec where 0 is the target value and 20 is the USL. In this case there is not a LSL.
     

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    May 4, 2007 at 5:06 pm #155724

    Dona
    ★ 10 Years ★
    Participant

    For a little better understanding of this process:
    A spinning object such as a tire will normally be not balanced meaning that there is more weight in one area so that as it spins it will cause vibration. The perfect balance is to have no unequal weight or 0. A value of zero means that the weight is equally distributed and no weight needs to be added to achieve proper balance.
    In my case balance has been set as a specification by the customer with a range of 0 mg – 20 mg. I am using these values as my USL and LSL.

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    May 4, 2007 at 6:09 pm #155728

    walden
    ★ 10 Years ★
    Participant

    Jon,
    As I stated in my earlier reply, 0 is not a LSL, it is a natural boundary. If you set LSL = 0 and have to calculate Cpk, as you approach your target of 0 (perfect balance) the calculated Cpk will also approach 0. If anybody tells you different, they don’t know what they’re talking about.
    Good luck,
    Chris

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    May 4, 2007 at 6:38 pm #155729

    Rob R.
    ★ 10 Years ★
    Member

    Jon, Chris nailed it for you. You can simply leave off the LSL since you can’t have a negative value. In Minitab, you can choose the Options button under capability and set a ‘Target’ value of 0, leaving you with a USL of 20 and a Target of 0.

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    May 4, 2007 at 8:01 pm #155731

    Steven Bonacorsi
    ★ 10 Years ★
    Member

    Jon,
    If your using Minitab for your capability analysis, follow the advice Chris gave, and leave the LSL value blank and check the box next to LSL that says LB (Lower Boundary).
    Hope that helps,
    Steven Bonacorsi

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    May 4, 2007 at 8:14 pm #155732

    GrayR
    ★ 10 Years ★
    Participant

    In Minitab, you can’t leave the LSL blank if you want to show it as a boundary.  If ‘0’ is the boundary, you enter it in the LSL cell, and check “Boundary”.  Minitab does not consider the boundary in the capability analysis — it returns a *.
     
     

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    May 4, 2007 at 8:49 pm #155733

    Steven Bonacorsi
    ★ 10 Years ★
    Member

    Thanks Gary,
    Had a brain fart there! :-)
    Steven Bonacorsi

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    May 10, 2007 at 1:05 pm #155928

    Russell
    ★ 20 Years ★
    Member

    The advice is sound and accurate.
    Just something to note is that if you go back and look at the data in a grahical format (distribution) it will be skewed. This is ‘normal’ for data that has a boundary / target of 0.
    Do not expect to see a standard bell curve for the data distribution.

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