Skip Lot Sampling - How to Perform Skip-Lot and Chain Sampling

[Montserrat P]

Hello:
I would like to know if somebody have experience related with the use of the Skip Lot Sampling Technical. Our company want to implant this technical in our process.

I will appreciate a lot all kind of information related with it.

[CarolX]

I have used skip lot sampling in the past, but it was so long ago, my tiny little mind does not recall how we did it.

Might I suggest using MIL-STD-105, and work into a reduced sampling plan?

Hello:
I need to use Skip Lot Sampling also for a new project....If somebody fromj you have experience from it, please let Montserrat know....

[This message has been edited by Marc Smith (edited 15 August 2000).]

[Marc]

Skip Lot is really Dock-to-Stock. In Dock-to-Stock, sooner or later your system should require a random or structured check.

A typical skip lot would be:

Inspect 3 lots. If OK, Dock-to-Stock for 5 shipments. Check the 6th shipment. If OK, Dock-to-Stock for 5 more shipments.

Continue as long as no nonconformances are identified.

If at any time time a nonconformance is identified, the inspection frequency is tightened - you define what 'tightened' is - such as inspect 3 shipments before returning to skip lot. Some systems are also set up to increase sample size when a nonconformance is identified.

Has anyone used AQL and skip lot systems to satisfy the Incoming Inspection requirements for QS-9000? We are considering using this as an interim measure for suppliers who are not yet certified, but our auditor is making grumpy noises.

[Montserrat P]

We are in the process to study the Skip Lot in one of our process, however, we have not thought about to implant it in the incoming inspection. Is the history of quality level of your suppliers so excellent to make the skip lot?
May I ask you, where did you find that information Sksp? I will appreciate to count on with more literature about it.

You should check out American Society for Quality - Statistics Division - Volume 4 "How to Perform Skip-Lot and Chain Sampling" by Kenneth Stephens.

The incoming quality varies - some suppliers are very good and skip lot will work; others will require constant incoming inspection. Question is, if the supplier has been shown to have adequate quality levels based on past history, but is not yet certified - will we be OK for our QS audit or is skip lot simply not allowed for QS incoming inspection? Hopefully our suppliers will be certified soon, but in the interim we need to do something and the cost of inspecting every shipment isn't justified for our good suppliers that are not yet certified.

You might check out Juran's Quality Control Handbook; he has a couple of pages on skip-lot schemes as does Montgomery in "Introduction to Statistical Quality Control." Also, you might search the web; seems like I found some good (and free) information of sampling a year or so back.

Good luck.