Understanding Special Cause and Common Cause Variation in Statistical Process Control

Common cause and special cause variations are both types of variation that can occur in a process. Common cause variations are expected fluctuations that follow the usual pattern of a process, maintaining its stability. On the other hand, special cause variations cause significant disruptions and instability, indicating an erratic process.

Detail about Common Cause Variation:

Common cause variation is the inherent variation found in a stable process. These variations are an intrinsic part of a system, measurable, expected, predictable, continuous, and consistent. Significant changes are typically necessary to address them.

A common cause of variation is when a task slightly deviates from its average completion time due to factors like wear and tear, computing delays, and measurement inaccuracies.

The benefit of Common Cause Variations is that common cause variations are always present and can be quantified using statistical methods to establish a standard for normal variation. These variations can be accommodated within the control limits of a control chart.

Detail about Special Cause Variation:

Special cause variations, also known as "assignable causes," are unforeseen anomalies significantly impacting a process. These deviations are atypical, immeasurable, and unpredictable.

Typically, the causes mentioned above stem from a particular alteration in the procedure, leading to a chaotic issue.

Special cause variations are linked to specific defects in a system. Adjusting components, methods, or processes can prevent their reoccurrence.