"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

"Since it has been proven to be easier and cehaper to operate existing processes up to their full potential than to upgrade and reenginer processes, any attempt to discuss process improvement that does not take advantage of process behavior charts to operate processes predictably is inherently flawed and incomplete." — Donald J. Wheeler, Reducing Production Costs

“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