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About this lesson
A stable process is one in which only random variation exists. A Lean Six Sigma team must eliminate sources of instability before attempting to improve the normal process performance.
When to use
Lean Six Sigma team will determine whether the process is stable during the Analyze phase. Based upon this analysis, the team will select the appropriate improvement strategy. The team will then need to test the improvement to ensure that the new process is stable.
A process is stable if there are no special cause variations present. Although the process is stable, it will still be subject to common cause variation. Most improvement strategies for a process are to remove the sources of special cause variation, and if that does not achieve the desired performance, then the process must be fundamentally changed to reduce the common cause variation that is inherent in the process design.
Common Cause Variation
Common cause variation is the normal random variation that is associated with any physical system. It is always present. Because it is always present and random in nature, it can usually be modeled with a normal distribution. That means that this variation is predictable and the process controls and management should plan for this level of variation and ensure the process is robust enough to accommodate this variation. Common cause variation can only be reduced by fundamentally changing the process.
Special Cause Variation
Special cause variation is not random, but rather it can be traced to a unique and unpredictable cause. It variation can be removed from a process by controlling for the conditions that enabled the unique cause to occur. Special cause variation leads to process instability because it cannot be modeled and predicted. Therefore, the occurrence cannot be planned for.
An interesting aspect of special cause variation is that sometimes the special cause can create unexpected and unsustainable good performance. This often occurs when a process change is introduced and everyone is watching every step of the process closely. The best people, the best equipment, and careful oversight is occurring. After the introduction, the normal process performance settles in and the variation is much higher than was experienced during the introduction. That excellent performance was due to the special cause of the extra oversight.
Hints & tips
- Make sure you understand whether the variation you see is due to special cause or common cause because the improvement strategy for each is totally different.
- Some people chase common cause variation as if it were special cause variation. This inevitably leads to tampering and often drives a stable process into a condition of instability.
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