Common Cause and Special Cause

Businessman looking at mainframe - Common cause and special cause variation

Common Cause and Special Cause in Statistics: Understanding Variability

Statistics is a powerful tool for analyzing data and making informed decisions, but to do so effectively, it’s essential to understand the sources of variability within a process or system.

In the realm of statistical process control, two fundamental concepts come into play: common cause and special cause variation. These concepts help us differentiate between the everyday fluctuations in a process and the exceptional, identifiable factors that can lead to significant deviations from the norm.

Common Cause Variation

Common cause variation, often called random variation or systemic variation, is the inherent variability in any process. It is the everyday, expected variation that occurs when a system is stable and operating under normal conditions.

This type of variation is the result of numerous factors and interactions within a process, and it cannot be traced back to a specific source. Common cause variation is, in a sense, the “background noise” of a process.

Key characteristics of common cause variation include:

  1. Inherent to the Process: Common cause variation is an inherent part of a process and will always exist to some degree.

  2. Consistent Patterns: It typically follows consistent, predictable patterns, often resembling a bell-shaped curve (a normal distribution).

  3. Random and Unpredictable: It is random in nature and cannot be attributed to any specific factor or event. This makes it difficult to control or eliminate entirely.

  4. Small Fluctuations: Common cause variation results in small, manageable fluctuations around a process’s mean or average value.

Examples of common cause variation can include minor temperature fluctuations in a manufacturing process, small variations in delivery times, or slight variations in the weight of identical products produced on the same assembly line.

Special Cause Variation

Special cause variation, also known as assignable variation or non-random variation, is the opposite of common cause variation. It represents variability in a process that can be traced back to specific, identifiable causes. Unlike common cause variation, which is inherent to the process, special cause variation is due to external factors or events that disrupt the system’s normal functioning.

Key characteristics of special cause variation include:

  1. Identifiable Causes: Special cause variation can be linked to specific events, actions, or factors that are not part of the usual operation of the process.

  2. Erratic Patterns: Unlike the consistent patterns of common cause variation, it often exhibits erratic and unpredictable patterns.

  3. Large Fluctuations: Special cause variation results in significant deviations from a process’s mean or average value.

  4. Unusual Events: Examples of special cause variation can include equipment breakdowns, power outages, errors in data entry, or major shifts in market demand.

Differentiating Between Common Cause and Special Cause Variation

Distinguishing between common cause and special cause variation is crucial in process improvement and quality control. Understanding the source of variability in a process allows organizations to take appropriate actions.

Here are some guidelines for differentiation:

  1. Data Analysis: The first step is to collect and analyze data. If the variation observed falls within the expected range of common cause variation, it is likely due to inherent process variability. However, if the data points exhibit patterns or values that deviate significantly from the norm, special cause variation may be present.

  2. Statistical Tools: Various statistical techniques, such as control charts, can be used to monitor processes and identify abnormal data points that suggest special cause variation. Control charts help in distinguishing between natural process variation and unusual occurrences.

  3. Root Cause Analysis: When special cause variation is suspected, a thorough root cause analysis is essential. This involves investigating the specific factors that contributed to the variation and taking corrective actions to prevent its recurrence.

  4. Process Control: Once special cause variation is identified and addressed, process control measures can be put in place to minimize the risk of future occurrences.

How can we Minimize Common Cause Variation?

Minimizing common cause variation is a key goal in statistical process control and quality improvement. While common cause variation is inherent to any process and cannot be completely eliminated, there are several strategies and approaches that can help reduce its impact and maintain greater process stability. Here are some ways to minimize common cause variation:

  1. Understanding the Process:
    • A thorough understanding of the process is the first step in minimizing common cause variation. You need to know how the process operates, what factors affect it, and the expected sources of variability.

  2. Process Control and Monitoring:
    • Implement process control tools, such as control charts, to continuously monitor the process. Control charts help in distinguishing between common cause and special cause variation. They visually represent the process’s performance over time, making it easier to detect trends or shifts.

  3. Standard Operating Procedures (SOPs):
    • Develop and maintain standardized operating procedures for the process. SOPs ensure that everyone involved follows the same methods and practices, reducing variability in human factors and operational choices.

  4. Training and Skill Development:
    • Invest in the training and skill development of employees involved in the process. A well-trained workforce is less likely to introduce unnecessary variability through errors or inconsistent practices.

  5. Reducing Equipment Variability:
    • Regularly maintain and calibrate equipment to minimize common cause variation associated with machinery or tools. Well-maintained equipment is more likely to produce consistent results.

  6. Statistical Process Control (SPC):
    • Use statistical techniques to understand the inherent variability in the process. By analyzing the process’s capability and identifying areas with excessive common cause variation, you can make data-driven decisions to improve it.

  7. Lean Six Sigma:
    • Implement Lean Six Sigma principles to identify and eliminate waste and non-value-added steps in the process. This can help streamline operations and reduce variability.

  8. Design of Experiments (DOE):
    • Use DOE methodologies to study the impact of various process factors on variability systematically. This approach can help optimize processes and identify which factors have the most significant impact on common cause variation.

  9. Process Improvement Teams:
    • Form cross-functional teams to focus on process improvement. Teams can work together to identify sources of common cause variation, develop solutions, and ensure continuous process optimization.

  10. Data-Driven Decision-Making:
    • Make decisions based on data and evidence rather than intuition. Data-driven decisions allow for a better understanding of the process’s performance and the identification of areas where common cause variation can be reduced.

  11. Feedback Loops:
    • Establish feedback loops to ensure that lessons learned from past performance are used to make continuous improvements. Regularly review and update process documentation, procedures, and best practices.

  12. Benchmarking:
    • Compare your process performance to industry benchmarks and best practices. Benchmarking can help identify areas where your process may be underperforming and experiencing excessive common cause variation.

  13. Employee Involvement and Suggestions:
    • Encourage employees to provide feedback and suggestions for process improvement. They often have valuable insights into daily operations and can help identify and address common cause variation.

Minimizing common cause variation is an ongoing effort requiring a systematic process improvement approach. Organizations can reduce variability and enhance their processes’ overall quality and performance by consistently monitoring, analyzing, and making data-driven adjustments.

How does Common Cause and Special Cause Apply in Six Sigma Projects

Common cause and special cause variation are fundamental concepts in Six Sigma, a structured and data-driven methodology for process improvement. Understanding these concepts is crucial for identifying, analyzing, and addressing variations within processes to reduce defects and improve overall quality. Here’s how common cause and special cause apply in Six Sigma projects:

  1. Defining the Problem (Define Phase): In the Define phase of a Six Sigma project, the team identifies the problem that needs to be addressed. At this stage, it’s essential to differentiate between common cause and special cause variation. Common cause variation represents the inherent variability in the process, while special cause variation signifies exceptional factors causing deviations from the norm. This distinction helps in setting realistic improvement goals and understanding the scope of the project.

  2. Data Collection and Analysis (Measure Phase): The Measure phase involves collecting data to quantify the performance of the process and determine its capability. Six Sigma practitioners use statistical tools and control charts to identify patterns in the data. Control charts help distinguish between common cause and special cause variation. Common cause variation is typically represented by data points within control limits, while data points beyond these limits suggest special cause variation.

  3. Root Cause Analysis (Analyze Phase): Once special cause variation is identified in the Measure phase, the Analyze phase focuses on determining the specific causes of these exceptional variations. Root cause analysis techniques, such as the “5 Whys,” Fishbone diagrams, or Failure Modes and Effects Analysis (FMEA), are employed to understand the underlying factors responsible for special cause variation. Addressing these root causes is critical for process improvement.

  4. Improvement Actions (Improve Phase): In the Improve phase, the Six Sigma team devises and implements solutions to eliminate or mitigate the root causes of special cause variation. Improvement actions are carefully planned, tested, and validated to ensure that the process becomes more stable and predictable.

  5. Monitoring and Control (Control Phase): The Control phase is about sustaining the improvements made during the project. Common cause variation is continuously monitored through control charts, and process performance is measured against the new standards. The control plan, established in this phase, ensures that the process remains in control and that deviations due to common cause variation are promptly identified and addressed.

  6. Continuous Improvement: Six Sigma is inherently focused on continuous improvement. Common cause variation is always present but can be further reduced and managed through ongoing efforts. Teams conduct periodic reviews and data analysis to detect changes in process performance and address any new sources of special cause variation.

Six Sigma projects involve a structured approach to addressing both common cause and special cause variation. While common cause variation represents the natural variability in a process, special cause variation results from specific, identifiable issues.

A Six Sigma project aims to minimize both types of variation to improve process performance and quality. This requires a combination of data analysis, root cause analysis, process improvement efforts, and ongoing monitoring to ensure that the improvements are sustained over time.


In the world of statistics and quality control, understanding the concepts of common cause and special cause variation is vital for making informed decisions and improving processes. Common cause variation is the inherent, expected variation in a process, while special cause variation represents unusual and identifiable sources of variability. By distinguishing between these two types of variation, organizations can work towards greater process stability, predictability, and overall quality improvement.

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