Statistical process control (SPC)
Beginner
Keeping a running chart of each measurement and stopping to investigate when one looks unusual.
Novice
Plotting measurements such as film thickness or line width on control charts with a center line and limits, typically 3 standard deviations away. A point outside the limits signals that something has changed.
Expert
Fab data is nested (sites within wafers within lots), so limits must reflect the right variance component, or charts either cry wolf on known lot-to-lot shifts or miss within-wafer drift.