Tampering in the context of a controlled process is adjusting the process on the basis of outcomes which are within the expected range of variability. The net result is to re-align the process so that an increased proportion of the output is out of specification. The term was introduced in this context by W. Edwards Deming, and he was a strong proponent of using control charts to avoid tampering.
W. Edwards Deming (1994) The New Economics for Industry, Government, Education, 2nd edition, Massachusetts Inst Technology. ISBN0-911379-07-X (Chapter 9.)
Deming, W. Edward (1986), Out of the Crisis, MIT Center for Advanced Engineering Study, 327–32. (2000 edition: ISBN0-262-54115-7)
Gitlow, Howard; Gitlow, Shelly; Oppenheim, Alan; Oppenheim, Rosa (1989), Tools and Methods for The Improvement of Quality, CRC Press ISBN0-256-05680-3