Cumulative sum chart: the tabular form keeps separate upper and lower sums, each adding the deviation from target minus a slack value k and resetting at zero, and signals when a sum passes the decision interval h. A common design is k of 0.5 sigma and h of 4 or 5 sigma, which catches a 1 sigma shift in about 8 to 10 samples on average, against about 44 for a plain individuals chart. The point where the sum starts climbing also estimates when the shift began.
CUSUM upper sum went over h on lot 4417, and it started climbing right after Tuesday's gas cylinder swap.
The CUSUM's turning point is the cheapest root-cause clue you will get: check the tool log at that timestamp before anything else.
Also heard as
- cumulative sum
- CUSUM chart
Related terms
EWMA
Exponentially weighted moving average: a chart or controller that blends each reading with the past to catch small, slow shifts.
Shift
Sudden, sustained step change in a process mean, often after a maintenance event, material change or recipe edit.
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
Special cause
Specific, identifiable source of variation that is not part of the process's normal behavior and shows up as a chart signal.