A special cause is a distinct event that pushes the process outside its usual pattern: a worn tool, a new reagent lot, a miscalibrated sensor, a recipe edit, an untrained operator. Control charts are built to detect special causes through points beyond the limits or run-rule patterns. The expected response is to find the cause, fix it and prevent its return, then annotate the chart so its history explains itself.
Special cause on the overlay chart traced to the reticle swap at 0300. Annotated it and released the lots after rework.
Write the cause on the chart the day it happens. Six months later nobody remembers why lot 4471 ran high.
Also heard as
- assignable cause
Related terms
Common cause
Background variation built into a stable process from many small sources, present all the time.
OOC
Out of control: shorthand for a point or chart that has violated a control limit or run rule.
Western Electric rules
Standard set of run rules that flag nonrandom patterns on a control chart, not just single points past the limits.
OCAP
Out-of-control action plan: the scripted checklist that tells operators exactly what to do when an SPC chart fires.
Excursion
Sudden departure of yield, defect counts or a key parameter from its normal band, big enough to trigger holds and an investigation.