natural bounds

The inherent range of variation in a process, as determined by process performance measures.

Natural bounds are sometimes referred to as “voice of the process.”

Techniques such as control charts, confidence intervals, and prediction intervals are used to determine whether the variation is due to common causes (i.e., the process is predictable or stable) or is due to some special cause that can and should be identified and removed. (See also “measure” and “process performance.”)