A common cause variation refers to the variation in a process that is caused by numerous small and random factors, none of which can be attributed to a single major cause. This type of variation is inherent in any process and is also known as natural variation or random variation.
Here's a breakdown of the concept:
1. Common cause variation: Common cause variation is also referred to as "within-group" or "random" variation. It represents the normal variability that occurs in a process due to the cumulative effect of various minor factors. These factors are inherent to the process and are not attributable to any specific root cause.
2. Many chance causes: Common cause variation arises from a multitude of small and random factors, often referred to as chance causes. These factors include random fluctuations, small measurement errors, inherent process variability, or minor changes in operating conditions. Each of these factors alone may have a minimal impact on the process, but their cumulative effect results in the overall variation observed.
3. Not traceable to a single major cause: Unlike special cause variation, which can be traced to specific factors or events, common cause variation cannot be attributed to a single identifiable source. It is the combined effect of multiple minor factors acting independently within the process.
An example to illustrate this concept could be the weight of packaged products on a production line. Common cause variation in this case may be due to factors such as variations in the material density, slight fluctuations in the calibration of weighing machines, or operator variances. These factors, although small individually, contribute to the overall variation observed in the weights of the packaged products.
It is important to understand and manage both common cause and special cause variation in order to improve processes and reduce variability. Identifying and addressing special cause variation can help eliminate specific sources of variation, while reducing common cause variation requires process improvement efforts aimed at minimizing the cumulative effect of multiple chance causes.