A shift is a step change in the process level that persists, unlike a single flyer. Typical triggers are preventive maintenance, a new consumable or chemical lot, a calibration change, a target edit or a tool swap. Large shifts trip rule 1 at once; shifts under about 1.5 sigma usually show up first as runs on one side of the centerline and are caught fastest by EWMA or CUSUM charts.
Thickness took a 6 angstrom shift right after the PM. Same scatter, new level, looks like the heater offset got reset.
Also heard as
- mean shift
- level shift
- step change
Do not mix it up with
- Drift: A drift is gradual and continuous; a shift is an abrupt step that then holds.
Related terms
Drift
Slow, steady creep of a process or gauge reading in one direction as something wears, depletes or ages.
Run of seven
Seven consecutive chart points on one side of the centerline, or steadily rising or falling, read as a process shift.
CUSUM
Cumulative sum chart: adds up deviations from target so that a small persistent shift builds into a clear signal.
EWMA
Exponentially weighted moving average: a chart or controller that blends each reading with the past to catch small, slow shifts.
Special cause
Specific, identifiable source of variation that is not part of the process's normal behavior and shows up as a chart signal.