What Is Acceptance Sampling?
Acceptance sampling is a statistical measure used in quality control. It allows a company to determine the quality of a batch of products by selecting a specified number for testing. The quality of this designated sample will be viewed as the quality level for the entire group of products.
A company cannot test every one of its products. There may simply be too high a volume or number of them to inspect at a reasonable cost or within a reasonable time frame. Or effective testing might result in the destruction of the product or making it unfit for sale in some way.
Acceptance sampling solves these problems by testing a representative sample of the product for defects. The process involves first, determining the size of a product lot to be tested, then the number of products to be sampled, and finally the number of defects acceptable within the sample batch.
Products are chosen at random for sampling. The procedure usually occurs at the manufacturing site—the plant or factory—and just before the products are to be transported. This process allows a company to measure the quality of a batch with a specified degree of statistical certainty without having to test every single unit. Based on the results—how many of the predetermined number of samples pass or fail the testing—the company decides whether to accept or reject the entire lot.
The statistical reliability of a sample is generally measured by a t-statistic, a type of inferential statistic used to determine if there is a significant difference between two groups that share common features.
A History of Acceptance Sampling
Acceptance sampling in its modern industrial form dates from the early 1940s. It was originally applied by the U.S. military to the testing of bullets during World War II. The concept and methodology were developed by Harold Dodge, a veteran of the Bell Laboratories quality assurance department, who was acting as a consultant to the Secretary of War.
While the bullets had to be tested, the need for speed was crucial, and Dodge reasoned that decisions about entire lots could be made by samples picked at random. Along with Harry Romig and other Bell colleagues, he came up with a precise sampling plan to be used as a standard, setting the sample size, the number of acceptable defects, and other criteria.
Acceptance sampling procedures became common throughout World War II and afterward. However, as Dodge himself noted in 1969, acceptance sampling is not the same as acceptance quality control. Dependent on specific sampling plans, it applies to specific lots and is an immediate, short-term test—a spot check, so to speak. In contrast, acceptance quality control applies in a broader, more long-term sense for the entire product line; it functions as an integral part of a well-designed manufacturing process and system.
- Acceptance sampling is a statistical quality-control measure that lets a company determine the quality of an entire product lot by testing randomly selected samples.
- When done correctly, acceptance sampling is a very effective tool in quality control.
- Developed during World War II as a quick fix for manufacturing, acceptance sampling should not replace more systemic acceptance quality control methods.
When done correctly, acceptance sampling can be a very effective tool in quality control. Probability is a key factor in acceptance sampling, but it is not the only factor. If a company makes a million products and tests 10 units with one default, an assumption would be made on the probability that 100,000 of the 1,000,000 are defective.
However, this could be a grossly inaccurate representation. More reliable conclusions can be made by increasing the batch size higher than 10 and increasing the sample size by doing more than just one test and averaging the results.