"A phenomenon will be said to be controlled when, through the use of past experience, we can predict, at least within limits, how the phenomenon may be expected to vary in the future. Here it is understood that prediction within limits means that we can state, at least approximately, the probability that the observed phenomenon will fall within the given limits."
This definition means that control is not equivalent to a complete absence of variation but rather that the system is in a state where variation is predictable within some fixed limit.
Shewhart also realized that frequent process-adjustment in reaction to non-conformance actually increased variation and degraded quality. That's why he expressed the fundamental rule of statistical process control in this way:
"Variation from common-cause systems should be left to chance, but special causes of variation should be identified and eliminated."
Without Statistical Process Control guidance, there could be endless debate over whether special or common causes were to blame for variability. This is crucial as the type of action needed to reduce the variability in each case are of a different nature.
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