ment, plan the flow of work, make statistical studies, and analyze production costs. In
particular, they rely heavily on a branch of mathematics called statistics and probability.
(For more about statistics and probability, see “Applied Mathematics.”)
How is math used in statistical process control?
Statistical process control (SPC) involves using statistical techniques to measure and
analyze the variations within a process. With SPC, industrial engineers monitor, control,
and, ideally, improve a process through statistical analysis. The four basic steps include
measuring the process, eliminating variables within the process to make it consistent,
monitoring the process, and, finally, improving the process to make the (usually better)
intended product. But it is not the answer to everything. All the SPC does is ensure that
the product is being manufactured and designed as intended. Thus, SPC does not tell
whether the design is good or bad, just if it’s made according to plan.
What is statistical quality control?
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 sam-
pling theory. (For more about sampling, see “Applied Mathematics.”)
Modern statistical quality control refers to using statistical techniques for mea-
suring 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 com-
pare, 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.
What is reliability?
Industrial engineers use another type of statistical technique called reliability, a sys-
tem that always produces the same results and that hopefully meets or exceeds its
specifications. A product is analyzed using the reliability function (or survivor func-
tion): the probability of a unit in a system that does not fail in a certain specified time
interval. If the unit does fail in a system, it means the end of the unit’s ability to per-
form the required function. This is determined by the failure distribution function, or
the probability of an item failing in a specific time interval. 343