Chasing the mean is what the floor calls tampering: every reading above target gets a knob turn down, every reading below gets a turn up, and the process starts oscillating around target with a wider spread than before. It often comes from a well-meant instruction to keep it centered with no control chart to say when a move is justified. A run-to-run controller with too much gain does the same thing automatically.
Day shift is chasing the mean on the coater, half the lots have a manual offset. Take the override away and let the chart decide.
A zigzag pattern that alternates above and below center, point after point, is the fingerprint of someone chasing the mean.
Also heard as
- knob-turning
- chasing the process
- chasing the target
Related terms
Tampering
Adjusting a stable process in reaction to common cause variation, which makes the output worse rather than better.
Knob
Any adjustable process parameter on a tool, such as RF power, pressure, gas flow, temperature or time.
Common cause
Background variation built into a stable process from many small sources, present all the time.
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.