Quality control has been around for a while in a crude sense. When a certain product was manufactured, and consumers chose that product, the makers would try to improve the quality of the product or lower its price. The improvement of the quality did not stop with the product, but also included the process for making the product. But the use of mathematics was minor in early quality control. It was not until the 1920s that statistics was applied to industry and quality control, mainly because of the development of sampling theory. (For more about sampling, see “Applied Mathematics.”)

Modern statistical quality control refers to using statistical techniques for measuring and improving the quality of processes; it is often broken down into statistical process control (SPC, see above) and statistical quality control (SQC). Both terms are usually used interchangeably, although SQC has a broader focus than SPc. To compare, SPC is the application of statistical techniques to control a process, reducing variation so that performance remains within specific limits; SQC is the application of statistical techniques to control quality and includes acceptance sampling (inspection of a sample from a lot to decide whether to accept that lot) as well as SPC.