Double and-Multiple Sampling

Extensions of the single sampling plans to double and multiple sampling plans are also available. In a double sampling plan, after the first n1 samples have been inspected there are three choices depending on the number of defectives found : 

1. reject the lot, 
2. accept the lot, and 
3. draw a second sample of n2 items. 

If choice (3) is made the final acceptance decision is made on combined sample of nl + n2 items. A multiple sampling plan operates in the same way, but with more than two samples. Double and multiple sampling plans reduce inspection costs because many accept/reject decisions are made based on the first sample which is smaller than that of the single sampling plan. However, single sampling plan is more common and easy to use. Details of multiple sampling plan are found in references.