Accounting for process variation is vital in Lean Six Sigma. The measurement phase of the LSS process produces the data that shows the amount of variation in the process. Once identified, the goal is then to manage and reduce this variation using Lean Six Sigma tools.
Basic Sources of Variation
There are two basic sources of variation, frequently known as common cause and special cause.
Factors that are a normal part of the manufacturing process, but are acting at random and independent of one another, create common cause variation. Typically, the key elements of the system – materials, equipment, people, environment, and methods – are the source of the variation. With only common cause variation at work, over time the outputs form a stable distribution.
Special cause variation occurs when a non-random event leads to an unexpected change in the manufacturing process output. Because the effect is intermittent, when special cause variation is present, the manufacturing process becomes unstable, and by virtue, unpredictable. Detecting and removing the special-cause variation allows manufacturers to bring the process under control.
Why Account for Variability
No one disagrees that accounting for variability in Lean Six Sigma is important. However, differentiating between common and special cause is often in dispute.
As I state in my book, The Quality Engineering Handbook,
The basic rule of statistical process control is variation from common-cause systems should be left to chance, but special causes of variation should be identified and eliminated.
To eliminate them requires investigation to determine what is different or changed in the process. This isn’t always immediately evident. With special-cause variation certain factors exist that may affect the process performance in a specific instance, as compared to performance in other situations.
Dealing with common-cause variation, on the other hand, requires a different mindset and group of tools. When it is present, it is necessary to look at the overall process to determine how to reduce variation and improve the process.
Accounting for variability in Lean Six Sigma allows you to identify the factors that affect performance. Once done, the DMAIC process enables you to uncover the underlying causes of defects and variations in order to implement sustainable improvements.