Miguel Hernandez

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    Miguel Hernandez


    First of all, I made a mistake, I confuse LCL and ULC from control charts with the LSL and LSL (sorry by that).

    I want to measure the Process capability index, where C_p = (USL – LSL)/6s, thus I need to get USL and LSL.
    They do not exist, as I told you. 97% +/-3% is the only datum I have from the customer.

    My yield is calculated as follows:

    DPO = defects / (numbers of units processed * Number of opportunities per unit) = 36/(259*2) = 0.06950

    Yield = (1 – DPO)*100 = 93.050% (actual yield)

    -where one defect on the product or a delay AND the product is considered totally defective-
    The number of products generated per week is a variable number (in the formula 259 is a sample taken from 25 weeks, as well as 36 defects).

    As you can see, in order to apply the formula of Process capability index, I need these spec limits. I can get standar deviation and average from my sampled data of 25 weeks.
    And also, the Ppk (long term capability). Finally, this is the question:

    Is it posible to get them (LSL and LSL) from the yield? If so, how?
    In other case, can I set them up? in which basis or based on what?

    Mass: Hope the explanation lightens the question.
    Kevin: I do not know ImR charts, I will look for them.
    Chris: Thanks for clarifying my error.

    Thanks for you support and answers, guys!
    Let me know if you need more details, please.

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