Variation

When a process is executed repeatedly, its outputs are seldom identical. For example, when a gun is successively fired at a target, as in the figure below, the bullets will not all pass through the same hole.

This lack of repeatability is caused by the variation or variability in the process. If these causes are understood, then this can lead to the development of solutions to reduce the variation in the process and result in more consistent products which require less inspection and testing, have less rejection and failure, cost less to build, have more satisfied customers and are more profitable.

Within any process there are many variable factors, as indicated above, each of which may vary a small amount and in a predictable way, but when taken together result in a degree of randomness in the output. These seemingly uncontrollable factors are called common causes of variation. Special causes of variation are unusual occurrences which come from outside the normal common causes.

Variation is not simple to measure, as by its nature is random and individual events cannot be predicted. Despite this, a degree of measurement can be achieved by looking at how a number of measurements group together, in terms of their centering and their spread.

Variation

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