In 1924, Dr. Walter Shewhart developed a new paradigm for managing variation. He concluded that although everyone and everything will always vary, e.g, no two people or things will ever be exactly alike, groups of things from a constant system of causes tend to be predictable.

Broader Description of Variation – American Society for Quality

Variation represents the difference between an ideal and an actual situation.

An ideal represents a standard of perfection—the highest standard of excellence —that is uniquely defined by stakeholders, including direct customers, internal customers, suppliers, society and shareholders. Excellence is synonymous with quality, and excellent quality results from doing the right things, in the right way.

W. Edwards Deming remarked that a failure to understand the difference between common and special causes of variation could result in 95% of changes resulting in no improvement.  A common example would be New Year resolutions. What percent of resolutions result in sustained improvement?

Shewart/Deming Terms	More Common Terms, Phrases, Examples
Common cause (chance)	Action that produces results that are considered usual, ordinary, normal, expected, or routine.  The majority of variation is common.
Special cause (assignable)	Action that produces results that are considered unusual, abnormal, temporary, unexpected, out of the ordinary, or an outlier. A special cause can be temporary or can indicate a shift to a new norm.
Stable system (contains only common causes)	Result that is considered routine, habitual, repetitive, predictable, i.e., If you always do what you always did, on average, you will usually get what you always got.
Unstable system (contains both common and special causes)	Result that is considered unpredictable and begs the question: “Who knows what will happen next?”
Mistake 1 – Treating a common cause as a special cause	The value of a mutual fund or stock goes down within a predictable range (see stable system) and you sell.
Mistake 2 – Treating a special cause as a common cause	You’re unusually late one day to work because of an accident that is unlikely to recur and you change your commuting process as a result.