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Common Cause - Special Cause
SPC Control Charts are designed to differentiate between special cause variation and common cause variation. In order to understand the importance of this and the implication for control, this lesson explains and illustrates the difference.
When to use
Common cause variation is always present in a process. Special cause variation is present in an unstable process. Whenever a process manager seeks to control a process, he or she needs to separate the variation into the appropriate categories so that appropriate actions can be taken.
SPC control charts are used to identify the differences between common cause variation and special cause variation. Once the process manager has determined the root cause for special cause variation and eliminated it, the remaining common cause variation is placed under statistical control in order to maintain a predictable process. For this reason, it is important that we clearly understand the differences between these two types of variation.
Common cause variation is the variation that is inherent in the design of the process characteristics. It is the typical variation between process operators, or the variation in equipment performance due to normal usage, it could even be differences in process performance due to environmental factors like heat, cold, or time of day. In one sense this variation is random, in another sense it is not. It is random to the extent that we don’t know if it will be higher or lower than the average value. However, the magnitude is not totally random. There is a normal range for the random variation that it will not exceed. Because of this, we can statistically model and predict the impact of the random variation on process performance. Finally, random variation can only be changed by making a fundamental process change so that the factors outlined earlier are different.
Special cause variation is not due to the inherent variation within the process design. It has as a unique root cause that is outside the inherent process operational characteristics. This root cause is not something the operator is normally monitoring and controlling. The occurrence of this root cause is unpredictable in timing and the magnitude of the impact is also unpredictable. That is why we say that the presence of a special cause variation leads to an unstable or unpredictable process. This unpredictability cannot be mathematically modeled and used to predict normal process performance. It is important to note that special cause variation is not always negative variation. Special causes may change important process parameters that reduce the inherent common cause variation within a process. This is referred to as the Hawthorne Effect – named after a research study conducted in Hawthorne, Illinois. In this study of worker productivity, it was determined that the primary influence on worker productivity was the special cause of having all the study specialists observing the worker’s behavior. Since they knew they were being studied, the worker changed their normal practice to be much more careful – but only when they were being watched.
Common Cause – Special Cause – Control Charts
Control charts are designed to differentiate between common cause variation and special cause variation. This is so the process managers and operators can remove the special cause variation which makes the process predictable. At that point, they can either maintain the normal variation or consider a fundamental change to the process to reduce the magnitude of the normal variation in the process. The operator should never chase the ups and downs of normal variation. Doing that will introduce process tampering which often leads to overcontrolling of the output and makes the overall performance even worse.
Hints & tips
- Identify and resolve special cause variation before attempting to make changes to the process and influencing common causes.
- Beware if tampering – it is an easy trap to fall into. It may give a short term benefit, due to the Hawthorne effect. But unless the process is fundamentally changed, it will go back to the original levels of common cause variation.…
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