Multiple factors influencing Spec minimum

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    Vendor is arguing that the minimum value we should accept is:

    1. Mean Spec – (3sigma) – (Mean storage loss + 3sigma) = Spec Minimum.

    They’re contradicting another specification used as reference which uses:

    2. Mean Spec – (3sigma) – (Mean Storage Loss) = Spec minimum.

    I think they’re right to use the 1st definition; however it’s defined more as minimum of minimum energy. Not certain how much argument leverage we have to get second definition.



    personally, I never use specifications which are a result of what a supplier (or my company itself) is able to deliver. Instead, I define a specification due to our product requirements. Hence, my spec does not (!) include statistical quantities, but reads “the measured value must be inside the interval [x1, x2]”. Hence, I don’t have any discussion on how the supplier likes to interpret the spec.

    Next, if I don’t want to perform a 100% incoming goods inspection, the supplier and I have to agree upon a sampling plan, at which AQL we are rejecting the lot, and how we proceed afterwards with the rejected lot. Doing these things upfront is important, because else you have many discussions about taking different samples from the same lot and arguing that the new measurement indicates that the hole lot is within specification. Once you understand how uncertainty and random sampling works you will see how the supplier transfers the costs to you.

    Finally, if I like to skip the incoming good inspection I demand that the supplier achieves a Cpk value, which exceeds a specified value. How this value is determined is not a matter of taste, but defined by using a formula. In addition, we agree that every lot has to satisfy this Cpk value and that we will use a lot (defined by my company) to reevaluate the Cpk value after a time X.

    I know that the discussions are hard and that no supplier likes this. So, if it is acceptable to us that we relax a specification, because we are able to tighten it for an other component, we do so. However, we do so because it is acceptable for our product and not because the requirements are hard for our supplier.

    Hope this helps.


    Timothy Lyall

    I rarely use certifications, but your post got me thinking about it.



    My comment is that techs should have a key role in negotiating contracts when specs are part of it. All too often it’s a deal between managers with limited technical expertise. How to measure compliance is left open to argument.

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