Common Cause Variation vs. Special Cause Variation

Attribute	Common Cause Variation	Special Cause Variation
Nature	Normal variation inherent in a process	Unusual variation caused by specific factors
Frequency	Occurs regularly and predictably	Occurs sporadically and unpredictably
Impact	Has a minor impact on outcomes	Can have a significant impact on outcomes
Control	Managed through process improvement	Requires investigation and corrective action

Definition

Common Cause Variation and Special Cause Variation are two types of variations that can occur in a process. Common Cause Variation refers to the natural variability that is inherent in a process and is to be expected. Special Cause Variation, on the other hand, is caused by specific factors that are not part of the normal process and can be identified and eliminated.

Characteristics

Common Cause Variation is random and unpredictable, occurring within the normal range of variation for a process. It is often referred to as "noise" in the system. Special Cause Variation, on the other hand, is non-random and can be traced back to specific causes. It is often referred to as "signal" in the system.

Impact

Common Cause Variation affects the overall performance of a process and can lead to small fluctuations in output. It is considered to be a part of the system and cannot be eliminated completely. Special Cause Variation, on the other hand, can have a significant impact on the process and can result in large deviations from the desired output. It is important to identify and eliminate special causes to improve process performance.

Detection

Common Cause Variation is typically detected through statistical process control methods such as control charts. These methods help to distinguish between common cause and special cause variation by analyzing the patterns of variation in the data. Special Cause Variation, on the other hand, can be identified through root cause analysis and investigation into the specific factors that are causing the variation.

Response

When Common Cause Variation is detected, the appropriate response is to focus on improving the overall process by reducing variability and making incremental changes. This may involve making adjustments to the process parameters or implementing process improvements. When Special Cause Variation is detected, the response should be to investigate and eliminate the specific causes of the variation. This may involve making targeted changes to the process or addressing specific issues that are contributing to the variation.

Examples

An example of Common Cause Variation in a manufacturing process could be variations in raw material quality or fluctuations in ambient temperature. These factors are inherent in the process and can lead to small variations in output. An example of Special Cause Variation could be a machine malfunction or operator error that results in a significant deviation from the desired output. Identifying and addressing these specific causes can help to improve process performance.