BUS606: Operations and Supply Chain Management

Statistical process control (SPC) is an effective method of determining whether a system has deviated from its normal statistical behavior by the use of control charts. Walter Shewhart first pioneered the use of statistical techniques when studying variations in product quality and consistency at Bell Laboratories in the early 1920's. SPC utilizes statistical tools in the form of control charts (such as Shewhart charts) to observe the performance of the production process in order to predict significant deviations that may result. Dr. Shewhart created concepts for statistical control and the basis for control charts through a series of carefully designed experiments. While he primarily drew from pure mathematical statistical theories, he understood that data from physical processes seldom produced a normal distribution curve (a Gaussian distribution, also known as a bell curve). Shewhart determined that every process displays variation: some display controlled variation that is natural to the process, while others display uncontrolled variation that is not always present in the process causal system.

SPC, most often used for manufacturing processes, is used in conjunction with experimental designs, process capability analyses, and process improvement plans to monitor product quality and maintain process targets. Six Sigma programs often depend on statistical process controls to provide their data with supportive information. If a process falls outside its preferred operating range, the statistical control can help you determine where and why the process failed.

Benefits of the SPC Method:

• Provides surveillance and feedback for keeping processes in control
• Signals when a problem with the process has occurred
• Detects assignable causes of variation
• Accomplishes process characterization
• Reduces need for inspection
• Monitors process quality
• Provides mechanism to make process changes and track effects of those changes
• Once a process is stable (assignable causes of variation have been eliminated), provides process capability analysis with comparison to the product tolerance

Capabilities of the SPC Method:

• All forms of SPC control charts
• Variable and attribute charts
• Average ($X$), Range ($R$), standard deviation ($s$), Shewhart, CuSum, combined Shewhart-CuSum, exponentially weighted moving average (EWMA)
• Selection of measures for SPC
• Process and machine capability analysis ($C{p}$ and $C{pk}$)
• Process characterization
• Variation reduction
• Experimental design
• Quality problem solving
• Cause and effect diagrams

SPC is used to monitor the consistency of processes used to manufacture a product as designed. It aims to get and keep processes under control. No matter how good or bad the design, SPC can ensure that the product is being manufactured as designed and intended. Thus, SPC will not improve a poorly designed product's reliability, but can be used to maintain the consistency of how the product is made and, therefore, of the manufactured product itself and its as-designed reliability.