Understanding Process Variation Types

Special-cause variation and common-cause variation are terms used in quality control to distinguish between different sources of variation in a business process.

• Common-cause variation refers to the natural or inherent fluctuations in a process that are always present and typically occur over time due to regular, predictable factors. This type of variation is usually stable and considered part of the system’s normal operating condition. For example, the daily fluctuation in customer foot traffic due to the time of year or holidays is a common-cause variation. It is predictable and does not indicate a need for immediate action unless it becomes excessive or exceeds acceptable thresholds.
• Special-cause variation refers to irregular, unexpected changes that arise from external factors, system malfunctions, or changes in the environment. These variations are not part of the process’s inherent characteristics and can signal problems that may require corrective action. An example of special-cause variation could be a machine breakdown in a manufacturing process. This breakdown introduces a variation in production that is not normal or expected and often requires intervention or repair to correct the issue.

Example of each variation:

• Common-cause variation: In a retail store, a typical variation in daily sales could be influenced by factors like day of the week or season. For instance, sales might be consistently lower on Mondays than on weekends, but this fluctuation is not caused by any specific issue—it’s simply part of the normal business rhythm.
• Special-cause variation: In a manufacturing environment, an unexpected equipment failure that causes a halt in production is a special-cause variation. It is an anomaly that requires immediate attention to get the production line back on track.

Part 2: Response to Post

The distinction made between common-cause and special-cause variation is quite accurate, and your examples are relevant to both types of variation. Common-cause variations, like weather delays, are typically a part of everyday life and don’t necessitate urgent action unless they significantly impact the process. As you mentioned, businesses that experience reduced profits and resort to downsizing may be experiencing a prolonged common-cause variation due to economic downturns or market changes that are expected but not alarming in themselves.

On the other hand, special-cause variations, such as running out of gas or a client going bankrupt, are unexpected and may require a reactive response. These types of variations can have a significant impact on the business, and the response often involves a more immediate corrective measure, like finding alternative ways to ensure deliveries or managing cash flow when clients fail to pay.

Both types of variations play a crucial role in the management of business processes. It is essential to identify when a variation is normal (common-cause) and when it is abnormal (special-cause) to effectively manage operations. Managing these variations effectively helps in maintaining stability while addressing any unexpected disruptions promptly.

This topic is relevant to quality management systems, where understanding the source of variation can improve decision-making and ensure more efficient processes.