What is Variation? Before diving into the specifics of common cause and special cause variation, let's first establish what variation means in the context of quality management. Variation refers to the differences or fluctuations observed in process outputs over time. These variations can arise from a multitude of factors, including environmental conditions, human interactions, equipment performance, and material properties.

Common Cause Variation: Common cause variation, also known as random variation or inherent variation, is the natural variability present in a process when it operates under stable conditions. It stems from factors that are inherent to the system and contribute to consistent, predictable fluctuations in outputs. Common causes are typically within the control of management and represent the normal operating conditions of the process.

Characteristics of Common Cause Variation:

Predictability: Common cause variation follows a stable pattern over time, allowing for statistical predictions within certain limits. Consistency: The variation observed under common causes is consistent and inherent to the process, representing its inherent stability. Gradual Changes: Changes in common cause variation occur gradually and are indicative of long-term shifts in the process.

Special Cause Variation: In contrast, special cause variation, also referred to as assignable cause variation, arises from specific, identifiable factors that are not inherent to the process. These causes are sporadic and unpredictable, leading to sudden shifts or outliers in process outputs. Special causes are typically beyond the control of regular management processes and require investigation and intervention to address.

Characteristics of Special Cause Variation:

Unpredictability: Special cause variation introduces unexpected deviations from the normal process behaviour, making it difficult to predict or anticipate. Occurrence: Special causes arise infrequently and are often associated with unique events or circumstances outside the usual operation of the system. Impact: Special causes can have a significant impact on process performance, leading to non-conformities, defects, or deviations from desired outcomes.

Managing Variation in Quality Management: Effectively managing both common cause and special cause variation is essential for ensuring quality and consistency in processes. Here are some strategies for addressing each type of variation:

Common Cause Variation: Focus on process improvement efforts aimed at reducing overall variability. This may involve implementing statistical process control (SPC) techniques, optimizing process parameters, and continuously monitoring performance metrics to identify long-term trends and opportunities for enhancement.

Special Cause Variation: Conduct root cause analysis to identify the specific factors contributing to the variation. Once identified, take corrective actions to eliminate or mitigate the underlying causes. This may involve implementing preventive maintenance programs, providing additional training to operators, or revising procedures to prevent recurrence.