Statistical Process Control

Statistical Process Control (SPC) is a diagnostic tool that allows you to determine “special” versus “common” causes of variation. Common causes of variation affect every process, and are why nothing is ever exactly the same. Special causes of variation occur when something happens that is not usually part of the process. SPC allows you to identify when these special causes occur so that you can eliminate them and bring predictability, or “control” to a process without overreacting to normal variability. SPC by itself does not change a process—you must have a reaction plan in place that tells you what to do when a special cause occurs.

By using a control chart you can graphically monitor a process variable and continuously perform a statistical test to determine if the mean has shifted, an indication that the process has changed. Control charts also identify an increase or decrease in process variability. The upper and lower limits of expected variability are calculated from historical data.

A common misconception about control charts is that they control a process. In reality, they are a powerful diagnostic tool to see if the process has shifted, and although they can be used to change the process when a special cause occurs, they are not the most efficient way to control a process in real time. Engineering Process Control (EPC) when used to its full capability will give you lower variability around a target. Click here to read a white paper by LWI consultant Althea D’Souza about some options to use control chart-like format to adjust your process.

If you want to learn more about SPC, you can purchase Quality With Confidence, written by LWI consultant Mike Petrovich and LWI founder Dr. Jeffrey Luftig. LWI also has the premier SPC training in the world available on site. Click here if you are interested in LWI customizing an SPC course at your site.