Tampering means correcting a process that is already in control, for example nudging a setpoint after every reading that lands off target. Because each adjustment reacts to random noise, it adds variation instead of removing it: compensating fully for each deviation from target roughly doubles the output variance, a result shown by the classic funnel experiment. The fix is to adjust only on a chart signal or through a properly tuned controller.
Stop tweaking the offset after every lot. That's tampering, and the Cpk drop since you started proves it.
Also heard as
- overadjustment
- overcontrol
- tweaking
Related terms
Chasing the mean
Shop phrase for adjusting a process after each reading to pull it back to target, which only adds variation.
Common cause
Background variation built into a stable process from many small sources, present all the time.
SPC
Statistical process control: using control charts and run rules to tell routine process noise from real changes.
Knob
Any adjustable process parameter on a tool, such as RF power, pressure, gas flow, temperature or time.