The tendancy 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 is taking action based on the belief that a common cause is a special cause.
Instead of reacting to individual points of data managers should examine the data within the context produced by the process over time. Then when the data signals that a special cause is likely, resources can be expended to examine the special circumstances that caused the significant result. The tool used to do this is the control chart.
"Tampering, over-reacting to variation, is a common method of increasing variation - and costs!" Brian Joiner (Leading for Rapid Improvement seminar notes)
Tampering (as variation) can also be defined as "additional variation induced by adjustments made in response to common cause variation" (Leading for Rapid Improvement seminar notes)
Online Resources:
Deming's Bio | Online Deming Resources | Deming Related Articles, reports, handbooks, etc.
Deming on Management