StATS: What is a special cause of variation?

A special cause of variation is a variation from the mean that has an assignable cause. When you have a special cause in your work process, you need to investigate immediately (while the trail is still warm).

When you spot a special cause:

  1. The first thing to do is control any damage or problems with an immediate, short-term fix. Be careful not to view this fix as a permanent solution or the process will never be improved.
  2. Once a quick fix is in place, search for the cause. Ask people in the process what was different that time. What was out of the ordinary? It might not have been much an unexpected emergency, a change in schedules, or new materials. The need for this sort of information is part of the reason for collecting very complete data the first time around, noting details and traceability factors about a sample or recorded event.
  3. Once you have discovered the special cause, you can develop a longer-term remedy. Most special causes have a negative impact on the output of the process and need to be removed. Occasionally, a special cause can have a positive impact depending on the nature of the process. If this is the case, finds ways to capture and integrate it into the system.

Avoid these mistakes:


This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Control charts, Category: Definitions.