In manufacturing, quality is a measurement of making acceptable final products while meeting specific goals, which could include low rejected products, minimum waste materials or lower rates of raw materials. To provide these goals, process control and quality must be linked, so that the manufacturing equipment is operating within acceptable limits. Process control also needs to include software or equipment that can determine when the process is out of limits or heading that way, so that suitable corrections can be made.
The term "statistical process control" is often used for computerized control systems developed since the late 20th century. Statistical control is simply maintaining a manufacturing process within a set of limits designed to provide the best statistical chance of a good finished product. Mathematical statistics can be used to determine the range of limits for the different process controls. Running a series of laboratory experiments and small-scale operations can then confirm the best control limits. Once the statistical limits have been established, control engineers can design electronic instrumentation to provide proper process control and quality.
Interest in process control increased in the 1950s because high-volume manufacturing during World War II showed the need for better quality while increasing production rates. In addition, countries with lower production costs could export more globally and replace local suppliers, leading to an interest in process control and quality to reduce manufacturing costs. Smaller electronic components became cost-effective for using in control systems in the later 20th century. This led to development of "intelligent" control systems that could analyze manufacturing variables and make independent adjustments.
Process control and quality were highly linked by the end of the 20th century, and a large number of statistical methods were developed at that time, including Six Sigma, Lean Manufacturing, Total Quality Management (TQM), and others. All were attempts to build consistent methods for determining quality and ways to control it, including raw materials, energy use, and process safety costs. These systems were created to find ways to control quality throughout a process, rather than changing process variables without understanding the effect on the final product.
As energy and carbon-based fuel costs rose in the late 20th century, process control and quality evolved into using controls both for creating good products, and to minimize the impact of rising petroleum costs. At that time, many countries relied heavily on oil, gas and coal for raw materials and to produce energy, resulting in final product costs linked closely to carbon-based fuels. Along with increased interest in energy efficiency, manufacturers continued to refine process control as well as quality software and instrumentation to maximize the efficiency of any raw materials created from petroleum, natural gas or coal.