6.4.2.1: Walter Shewhart (1891-1967)

Dr. Shewhart was an American physicist, engineer and statistician. He is known as the father of statistical quality control and spent much of his career researching variation and is credited with the creation of the first control chart. His work focused around the need to reduce variation in order to improve quality. He is responsible for the concepts of assignable and common variation.

The processes that produce goods and services will all have some variation. It is intuitive that the more variation in processes, the poorer the quality will be. Assignable variation is the type of variation where the cause can be clearly identified and corrected or managed. An example of an assignable variation might be an error by an employee, a software glitch, or a tool breakage. Common variation, also referred to as chance variation, is the type of variation that is inherent in the process. It is generally to be expected, and not a cause of an error. Intuitively, if we reduce or minimize either variation, we will improve product quality.

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