Differentiate between common cause variation and special cause variation.

Differentiate between common cause variation and special cause variation.

• A. Common cause is random, inherent process variation; special cause is due to identifiable assignable factors.
• B. Common cause is due to identifiable factors; special cause is random.
• C. Common cause refers to measurement error; special cause refers to material defects.
• D. Special cause is the result of external events; common cause is internal drift.

Answer: (A)

Variation in a process comes from two sources: common cause variation, which is random and inherent to the process, and special (assignable) variation, which arises from identifiable factors outside normal operation. Common cause variation is the natural fluctuation you expect when a process runs under stable conditions—it's caused by many small, interacting elements in the system (machine, method, materials, environment, people). Because it’s built into the process, you can’t eliminate it entirely; you improve it only through fundamental changes to the system or design to reduce the overall variability. Special cause variation, on the other hand, comes from specific factors you can identify and remove—things like a faulty piece of equipment, a change in material, or an operator mistake. When a special cause occurs, the process behaves abnormally and is out of control; once the cause is addressed, the process typically returns to its normal, common-cause-driven variation level. That’s why describing common cause as random and inherent and special cause as arising from identifiable factors is the most accurate differentiation.

Variation in a process comes from two sources: common cause variation, which is random and inherent to the process, and special (assignable) variation, which arises from identifiable factors outside normal operation. Common cause variation is the natural fluctuation you expect when a process runs under stable conditions—it's caused by many small, interacting elements in the system (machine, method, materials, environment, people). Because it’s built into the process, you can’t eliminate it entirely; you improve it only through fundamental changes to the system or design to reduce the overall variability.

Special cause variation, on the other hand, comes from specific factors you can identify and remove—things like a faulty piece of equipment, a change in material, or an operator mistake. When a special cause occurs, the process behaves abnormally and is out of control; once the cause is addressed, the process typically returns to its normal, common-cause-driven variation level.

That’s why describing common cause as random and inherent and special cause as arising from identifiable factors is the most accurate differentiation.