Tampering with a process occurs when we respond to variation In the process (such as by “adjusting” the process) when the process has not shifted. In other words, it is when we treat variation due to common causes as variation due to special causes. This is also called “responding to a false alarm,” since a false alarm is when we think that the process has shifted when it really hasn’t. Edward Deming in his famous “Funnel Experiment” illustrated how tampering actually increases variation. Tampering has also been described by many as “Management’s Most Costly Error”
Tampering -Modifying a system by treating common cause variation as special cause variation. This always degrades system performance.
(Advanced Projects Institute TQM Dictionary)
Tampering – Taking action based on the belief that a common cause is a special cause. The tendency to take action, often leads to action without reason which causes more problems than it fixes. Dr. Deming stated that most variation (97% plus) was common cause variation not due to special causes. Tampering can also be considered a form of variation.
(Curious Cat Management Improvement Encyclopedia)