Control Plan issues

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    I have a supplier who has a long term Cp of around 1. His control plan says he checks less than 100% of his parts. I know for sure that this will results in defects being shipped. Is there a rule/formula I can use to assign control plans,based on Cp values. I have seen one put out by the AIAG, but this does not have any theory behind it. Help would be appreciated.



    This is a very interesting question. I think it can be approached in a few different ways. First – if the supplier is sampling less than 100% of the parts AND if the % of defectives is significantly high, then it may not be necessary to sample more parts. The reason being, there would already be significant opportunity for improvement. On the other hand if the % of defectives is acceptably low, then the question is what is the sampling strategy. I would contend that changing the sampling strategy a few times would be a good idea to validate there is no measurement bias.
    Second way of looking at it is, that complementary sampling strategy where the parts are received, may help expose issues with inspection at the suppliers end.
    A third perspective would be to verify is the inspection effective enough? If it is not, you may actually want to focus on improved inspection as opposed to increased coverage.

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