Understanding Variation and Its Causes

In any process, whether manufacturing, service, or administrative, there is always some degree of variation. Understanding the sources of this variation is crucial for process improvement and quality control. Variation is typically categorized into two main types: chance causes and assignable causes.

What are Chance Causes of Variation?

Chance causes, also known as common causes, are the inherent, random variations that are present in a process that is operating in a stable state. They are the result of many small, unavoidable factors that are always present. Imagine slight fluctuations in temperature, tiny variations in material properties, or the natural differences in how different operators perform a task within defined procedures. These causes are typically:

• Random and unpredictable individually.
• Numerous and small, with their combined effect creating a stable pattern of variation.
• Present even when the process is considered "in control" or stable.
• Difficult or impossible to eliminate completely without fundamentally changing the process system itself.

What are Assignable Causes of Variation?

Assignable causes, also known as special causes, are specific, identifiable factors that are not part of the normal process variation. They are typically sporadic events that cause larger, unpredictable shifts or patterns in the data. Think of a machine breaking down, a new, untrained operator being introduced, a batch of defective raw material, or a sudden change in environmental conditions. These causes are typically:

• Specific and identifiable.
• Cause larger deviations from the average compared to chance causes.
• Indicate that the process is unstable or "out of control".
• Often preventable or correctable once detected and diagnosed.

Analyzing the Statements on Variation

Let's evaluate each statement given in the question based on our understanding of chance and assignable causes:

Statement 1: Chance causes variations are preventable.

Chance causes are inherent random variations. While the overall system can be improved to reduce the *range* of common cause variation, the individual chance causes themselves are not typically prevented in the way assignable causes are. They are considered unavoidable background noise in a stable process. Therefore, stating that chance causes variations are "preventable" is generally considered INCORRECT in the context of routine process operation and management.

Statement 2: Chance causes are beyond the control of human hand.

This statement aligns with the nature of chance causes. They are the result of many small, random factors inherent in the process and are not typically controlled or eliminated by individual intervention. This statement is CORRECT.

Statement 3: Assignable causes of variation are preventable.

Assignable causes stem from specific issues like equipment failure, training problems, or material defects. Once these specific issues are identified, actions can be taken to address them, thus preventing their recurrence. This statement is CORRECT.

Statement 4: Assignable causes can be detected and measured.

Assignable causes typically cause the process output to deviate significantly from the norm, creating patterns (like points outside control limits or trends) that can be detected using statistical tools like control charts. The impact of these causes on the process variation can also be measured. This statement is CORRECT.

Based on the analysis, the statement that is INCORRECT is "Chance causes variations are preventable."

Revision Table: Chance vs. Assignable Causes of Variation

Additional Information on Managing Variation

Managing variation is a core aspect of quality management and process improvement. Statistical Process Control (SPC) is a methodology often used for this purpose. Control charts are a key tool in SPC used to distinguish between chance causes and assignable causes of variation.

• When a process shows only chance causes, it is considered statistically "in control" or stable. Efforts to reduce variation in this state require fundamental changes to the process system itself (e.g., new technology, different materials, re-engineering the process).
• When assignable causes are present, the process is "out of control". The first priority is to identify and eliminate these specific causes to bring the process back to a stable state, operating with only chance cause variation.

The ability to correctly identify the type of variation present is critical because different strategies are needed to address them effectively.