Every once in a while, people ask about acceptance sampling plans and I get all riled up. We all know (especially in this political season) that humans are addicted to their indignation high, so here’s your fix for today.

Back when defective products and services were considered inevitable, the military created a standard to try to control just how much defective product was allowable before it would be rejected. This is totally antithetical to the modern concept of quality (conformance to target) or Six Sigma (conformance to specification).  And yet, hard as it is to believe, companies are still using these plans. So what’s so bad about them, and how can you help a company move away from them?

You can kind of see the original purpose of acceptance plans. If we assume that defective product is inevitable, then we had better figure out a way for making sure we don’t get too many defective products. To this end, the creators of MIL-STD-105 (and its descendants, e.g., ANSI/ASQ Z1.4) used the concept of inferential statistics (based on the binomial distribution) to create a sampling scheme that could be used by a customer to determine if they should accept a shipment from a supplier. They constructed a series of tables and rules that would allow you to determine how large a sample to take, and the number of “bad” samples allowed before you would reject the shipment.

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