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This post was sent in by Susan Young: We are an OEM Mfg. that has 100$% inspection. We put a dock to stock in place and when it didn't take off, we pulled the procedure from our master document list. I believe dock to stock is an outdated tool and I would like to understand best practice of skip lot so that possibly we could get some parts through the process quicker.
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8 Posts
The Statistics Division has an ebook on Skip Lot Sampling available for $15 through ASQ Press, part of our How To Series:
#4 How to Perform Skip-Lot and Chain Sampling, 2nd Edition, by Kenneth S. Stephens
https://asq.org/quality-press/display-item?item=E0882
Another good reference is the NIST Engineering Statistics Handbook, which briefly outlines the process here.
https://www.itl.nist.gov/div898/handbook/pmc/section2/pmc27.htm
#4 How to Perform Skip-Lot and Chain Sampling, 2nd Edition, by Kenneth S. Stephens
https://asq.org/quality-press/display-item?item=E0882
Another good reference is the NIST Engineering Statistics Handbook, which briefly outlines the process here.
https://www.itl.nist.gov/div898/handbook/pmc/section2/pmc27.htm
5 Posts
It would be interesting to know why dock to stock didn’t “take off”.
In any case to implement skip lot sampling you follow a simple progression of steps. There is a lot of valuable literature about this, statistical analysis, etc. I’m giving you a practical method.
First, shift from 100% inspection to lot-by-lot sampling inspection. Start with a low AQL. Use one of the three common sampling plans, Z1.4, c=0, or Z1.9. Start in tightened inspection. (A low AQL and tightened inspection will help reduce the risk as you go through the progression.)
Follow the switching rules in the standard until you get to reduced inspection.
After some number of lots in reduced inspection, switch to skip lot and inspection 1 in 2 lots. After some number of lots switch to inspecting 1 in 4 lots.
Use the normal inspection sampling parameters for the skip lot inspection.
In any case to implement skip lot sampling you follow a simple progression of steps. There is a lot of valuable literature about this, statistical analysis, etc. I’m giving you a practical method.
First, shift from 100% inspection to lot-by-lot sampling inspection. Start with a low AQL. Use one of the three common sampling plans, Z1.4, c=0, or Z1.9. Start in tightened inspection. (A low AQL and tightened inspection will help reduce the risk as you go through the progression.)
Follow the switching rules in the standard until you get to reduced inspection.
After some number of lots in reduced inspection, switch to skip lot and inspection 1 in 2 lots. After some number of lots switch to inspecting 1 in 4 lots.
Use the normal inspection sampling parameters for the skip lot inspection.
