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Govt. Certified Six Sigma Black Belt

Basic Concepts

Dr. Walter Shewhart identified two sources of process variation:

• Chance variation that is inherent in process, and stable over time, and
• Assignable, or Uncontrolled variation, which is unstable over time – the result of specific events outside the system.

Dr. Deming relabelled chance variation as Common Cause variation, and assignable variation as Special Cause variation.

After experience with many types of process data, and supported by the laws of statistics and probability, control charts are used to plot data over time and identify both Common Cause variation and Special Cause variation.