Statistical process control

Statistical Process Control, or SPC is a method for achieving quality control in manufacturing processes. It was pioneered by Walter A. Shewhart and taken up by W. Edwards Deming with significant effect by the Americans during the 2nd World War to improve aircraft production. Deming was also instrumental in introducing SPC techniques into Japanese industry after that war.

The technique hinges on the observation that any manufacturing process is subject to seemingly random variations, which are said to have common causes, and non-random variations, which are said to have special causes. A common cause might be air movement in the manufacturing environment, which causes variations which are outside the control of manufacturing operatives. A special cause might be the fact that the operative has a hang-over. Management can usually determine special causes for manufacturing defects by consulting the workforce, but dealing with common causes is a management responsibility.

SPC relies on measuring variation in manufacturing output and setting control limits based on observations of variations arising solely from common causes. Output which is outside the control limits is ignored from the perspective of process quality improvement as being due to a special cause. The quality improvement process consists of varying the production process to achieve a smaller range of control limits. It has been shown that manufacturing processes can achieve control limits which are a tenth of the specified manufacturing tolerance. Such a process can then be said to achieve zero defects, and the reduction in wastage and inspection resources makes it more efficient than processes which do not employ SPC.