Adjusting a stable process in an attempt to improve the next result by compensating for or taking into account the deviation from the target of the previous result. Tampering, or over-adjustment of a stable process, actually increases the variation of the results. OR action taken to compensate for variation within the control limits of a stable system; tampering increases rather than decreases variation, as evidenced in the funnel experiment.
Tampering in the context of a controlled process is when adjustments to the process are made based on 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