When a process is being affected by special causes of variation, it is called "unstable", or "out of control". Removing special causes when they are harmful (which is most of the time) or integrating them when they are beneficial (which is rare) is an important part of process improvement.

It is easiest to deal with special causes if they are spotted early and the data used to identify them is timely. Tracking down special causes relies heavily on people's memories of what made that occurrence different from all the others. People may quickly forget any unusual circumstances that may have triggered the unusual variation.

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:

• Changing the process to accommodate the special cause. This usually adds cost and bureaucracy.
• Blaming individuals. Not only does everyone makes mistakes, but also chances are that the problem would have occurred regardless of individuals involved.
• Exhorting workers to simply "do better." People can only do as well as the system allows them to do.

It's important not to stop when you've eliminated special causes of variation. You're only halfway, at that point. The next thing is to reduce common cause variation via systematic process improvement. Follow this link to find out how: Responding to Common Cause Variation