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Causes of Variation in Quality

Variations in quality can stem from different sources. Understanding these causes helps in identifying and addressing quality issues effectively. Causes of variation are often categorized into common causes and special causes.

Common Causes of Variation

Common causes of variation are inherent in the production process. These are systematic, recurring sources of variation that often result from the design of the process, environmental factors, or operational practices. They typically represent the “noise” or baseline level of variability in a system. Reducing common causes usually requires improvements in the overall system design.

Examples of Common Causes:

  1. Poor Product Design
    Ineffective design can lead to inconsistencies and variations in quality because the product itself may be prone to issues, such as fragile components or difficult assembly requirements.

  2. Poor Process Design
    If the process layout or workflow is inefficient, it can lead to bottlenecks and quality issues due to inadequate handling or inconsistent operations.

  3. Unfit Operation
    Operations that are not well-suited for the task can lead to quality variation, as processes may not be executed in the best way.

  4. Unsuitable Machine
    Machines that are not designed or equipped for the specific tasks can introduce variability, as they may lack the precision or capabilities needed for high-quality output.

  5. Untrained Operators
    Operators who are not properly trained may make mistakes or be inconsistent in performing their tasks, resulting in product variations.

  6. Inherent Variability in Incoming Materials from Vendor
    Variability in the quality of materials received from suppliers can lead to inconsistencies in the final product, as each batch may perform differently.

  7. Lack of Adequate Supervision Skills
    Insufficient supervision can lead to lapses in quality control, as supervisors may not notice or correct issues in time.

  8. Poor Lighting
    Inadequate lighting in the workspace can lead to errors, especially in tasks that require visual precision, contributing to quality variation.

  9. Poor Temperature and Humidity
    Environmental factors like temperature and humidity can affect materials and processes, leading to changes in quality, especially in sensitive manufacturing processes.

Special Causes of Variation

Special causes of variation are unexpected, unusual factors that can be traced to a specific event or occurrence. Unlike common causes, these are not inherent to the process but are instead sporadic and often due to avoidable human or mechanical errors. Detecting and correcting special causes is typically more straightforward as they often have identifiable sources.

Examples of Special Causes:

  1. Negligence of Operators
    Mistakes or oversight by operators due to negligence can lead to significant quality issues that deviate from the usual process.

  2. Defective Raw Materials
    Receiving materials that are substandard or do not meet specifications can introduce sudden, unplanned variation in quality.

  3. Faulty Equipment
    Malfunctioning equipment can lead to defects, as it may not operate according to specifications, affecting the quality of the output.

  4. Improper Handling of Machines
    Incorrect use or mishandling of machinery can lead to unexpected variations, such as damaged components or inconsistent product features.

Conclusion

Understanding the difference between common and special causes of variation is essential for quality management. Common causes require long-term improvements to the system, focusing on overall process and product design, operator training, and environmental controls. Special causes, on the other hand, can often be identified and corrected quickly, as they are linked to specific incidents or errors. Addressing both types of causes can lead to a more stable and high-quality production process.

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