Statistical Quality Control (SQC) distinguishes between two main sources of variation in processes and products: chance causes and assignable causes. Understanding the difference between these two types of variation is essential for effectively managing and improving quality. Let’s explore the characteristics of chance causes and assignable causes, along with the key differences between them:

Chance Causes of Variation:

Chance causes, also known as common causes or random variation, are inherent to the process and arise due to natural fluctuations in materials, equipment, or environmental conditions. These variations are typically small and occur consistently over time. Examples of chance causes include minor fluctuations in temperature, humidity, or raw material properties.

Characteristics of Chance Causes:

1. Inherent to the Process: Chance causes are part of the normal variation observed in a stable process.
2. Predictable Patterns: Variations due to chance causes follow statistical distributions, such as the normal distribution or Poisson distribution.
3. Continuous and Gradual: Changes caused by chance variation tend to occur gradually over time, affecting process output in a consistent manner.
4. Addressed through Process Improvement: Managing chance causes involves implementing process improvements to reduce variability and enhance process stability.

Assignable Causes of Variation:

Assignable causes, also known as special causes or non-random variation, are attributed to specific factors or events that are not part of the usual process. These causes result in significant deviations from the expected output and require immediate investigation and corrective action. Examples of assignable causes include equipment malfunction, operator error, or sudden changes in raw material quality.

Characteristics of Assignable Causes:

1. External Factors: Assignable causes originate from factors external to the normal process operation, such as equipment failures or human errors.
2. Unpredictable Events: Variations caused by assignable causes are unpredictable and sporadic, leading to sudden changes in process performance.
3. Discrete and Isolated: Assignable causes often result in distinct deviations from the expected output, which can be identified through data analysis.
4. Addressed through Root Cause Analysis: Resolving assignable causes involves identifying the root cause of the deviation and implementing corrective actions to prevent recurrence.

Key Differences:

1. Nature: Chance causes are inherent to the process and result in predictable, gradual variations, while assignable causes are external factors that lead to unpredictable, sudden deviations.
2. Consistency: Chance causes produce variations that occur consistently over time, whereas assignable causes lead to sporadic and isolated deviations.
3. Control: Chance causes can be managed through process improvement efforts aimed at reducing variability, while assignable causes require immediate investigation and corrective action to eliminate the root cause of the deviation.

Conclusion:

In summary, chance causes of variation are inherent to the process and result in predictable, gradual fluctuations, whereas assignable causes are external factors that lead to unpredictable, sudden deviations. By distinguishing between these two types of variation, organizations can effectively manage quality and ensure the stability and consistency of their processes and products. Identifying and addressing assignable causes promptly is critical for preventing defects and achieving continuous improvement in quality performance.