Techniques for Reducing Variation
Reacting to common cause variation requires a different mindset and set of techniques than reacting to special cause variation. When special cause variation is present, it means that specific factors exist that can impact the process performance in a specific instance, as compared to performance in other situations. For example, weather problems, power outages, and traffic accidents represent special cause that can impact whether pizza is delivered to a customer within the appropriate amount of time.
In order to reduce variation due to special causes, Six Sigma project teams can put in place a procedure for detecting the presence of those factors in a timely fashion, and using contingency plans to minimize their impact. For example, when a delivery route is blocked due to accident or unexpected construction, a notification can go out to all dispatchers and driver so that another route is employed.
When only common cause variation is present in a process, it does not make sense to try to understand why performance was as it was in one specific instance. It is necessary to look at the process overall and determine how variation can be reduced and performance improved. While special cause may be a sign that a process is fundamentally sound but prone to impact by outside forces, common cause is a sign that substantial variation exists in the process itself and that it needs fundamental changes to meet the performance goals and to reduce variation.
The most common way to understand and impact common cause variation is to determine major factors that vary, such as department performing a function, time of day that the process is performed, or equipment used in the process. Graphing and data analysis techniques are used to separate out these different parts of the process, such as by evaluating performance separately for the day shift and the night shift or based on which type of product is being processed.
Once the factors affecting performance are identified, the DMAIC process can be followed to uncover root causes for defects and variation and to implement sustainable improvements.