Statistical process control: Difference between revisions

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'''Statistical process control''' ('''SPC''') is a method of [[quality control]] which employs [[statistics|statistical methods]] to monitor and control a process. This helps to ensure that the process operates efficiently, producing more specification-conforming products with less waste (rework or [[scrap]]). SPC can be applied to any process where the "conforming product" (product meeting specifications) output can be measured. Key tools used in SPC include [[run chart]]s, [[control chart]]s, a focus on [[Continuous Improvement Process|continuous improvement]], and [[Design of experiments|the design of experiments]]. An example of a process where SPC is applied is manufacturing lines.

Although SPC is normally thought of in industrial applications, it can be applied to virtually any process. Everything done in the workplace is a process. All processes are affected by multiple factors. For example, in the workplace a process can be affected by the environment and the machines employed, the materials used, the methods (work instructions) provided, the measurements taken, and the manpower (people) who operate the process—the Five M’s. If these are the only factors that can affect the process output, and if all of these are perfect—meaning the work environment facilitates quality work; there are no misadjustments in the machines; there are no flaws in the materials; and there are totally accurate and precisely followed work instructions, accurate and repeatable measurements, and people who work with extreme care, following the work instructions perfectly and concentrating fully on their work—and if all of these factors come into congruence, then the process will be in statistical control. This means that there are no special causes adversely affecting the process’s output. Special causes are (for the time being, anyway) eliminated. Does that mean that 100% of the output will be perfect? No, it does not. Natural variation is inherent in any process, and it will affect the output. Natural variation is expected to account for roughly 2,700 out-of-limits parts in every 1 million produced by a three-sigma process (±3σ variation), 63 out-of-limits parts in every 1 million produced by a four-sigma process, and so on. Natural variation, if all else remains stable, will account for two out-of-limits parts per billion produced by a true six-sigma process.

SPC must be practiced in two phases: The first phase is the initial establishment of the process, and the second phase is the regular production use of the process. In the second phase, a decision of the period to be examined must be made, depending upon the change in 5M&E conditions (Man, Machine, Material, Method, Movement, Environment) and wear rate of parts used in the manufacturing process (machine parts, jigs, and fixtures).

An advantage of SPC over other methods of quality control, such as "[[inspection]]," is that it emphasizes early detection and prevention of problems, rather than the correction of problems after they have occurred.