"A phenomenon will be said to be controlled [predictable] when, through the use of past experience, we can predict, at least within limits, how the phenomenon will behave in the future." — Walter A. Shewhart, Economic Control of Quality of Manufactured Product

“The process principle states that the sequential outputs of a process are subject to some degree of difference, whether we can measure those differences or not.” — Jim Lehner, The Virus of Variation