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Sampling Plan

A sampling plan is a procedure for selecting samples from a lot of material and using the results to make an accept or reject decision.  They can be used in manufacturing to help make product disposition decisions as well as in development to demonstrate that the product/process meets certain requirements.

Sampling plans are commonly used to make product disposition decisions.  Accept corresponds to releasing the lot and reject corresponds to placing the lot on hold.  In this case, one typically starts by defining the worse level of quality one considers acceptable for release, called the AQL.  One also specifies an even worse level of quality which should be consistently rejected, called the LTPD.  The sampling plan is then selected to satisfy these two requirements.  Failing the sampling plan then demonstrates that the AQL has been exceeded and that the lot is bad.  While passing the lot does not necessarily prove the lot is good, it does demonstrate that the lot is not too bad and at least is better than the LTPD.    

Sampling plans are also commonly used in validation and verification studies designed to demonstrate that process meets certain requirements.  Accept demonstrates the requirement are met.  Reject corresponds to failing to demonstrate the requirements are meet.  In this case, one typically starts by defining the level of quality one wants to prove is met, called the LTPD.  One also specifies a better level of quality one hopes to achieve and would like to a high chance of passing the validation study, called the AQL.  The sampling plan is then selected to satisfy these two requirements.  Passing the sampling plan then demonstrates that the LTPD has been meet and allows one to make a confidence statement such as with 90% confidence, the defect rate is below 1% defective.  Failing the sampling plan does not necessarily mean the product/process is bad but does mean it has failed to be demonstrated good.  One can at least conclude that the quality level must exceed the AQL for which the plan was designed to pass.