Cusum Chart in Excel

Cumulative Sum Charts Can Detect Small Shifts in a Process

Cusum charts can detect process shifts more rapidly than the XmR or XbarR. If a trend develops, it's evidence that the process has shifted.

Example of a Cusum Chart Created by QI Macros Add-in for Excel

cusum chart in excel

A Cusum Chart consists of:

  • A target which is either provided by you or calculated as the mean of your data.
  • Upper Cusum Limit and Lower Cusum Limit lines.
  • C+ and C- lines which represent the cumulative sum of deviations of successive sample means from a target. Use these lines to determine if your process is shifting.

Default values calculated in the Cusum Chart:

k (slack) = 1/2 of your StDev
(this is an estimated value, but can be changed)

FIR (Fast Initial Response) = 0
(This feature can be used to set an initial "head start" value to improve detection of an out-of-control condition at the start of the chart. If the process is in control, this initial head start has little effect; if the process is out of control, a signal will be given faster with a non-zero value than with a zero value)

h (threshold value or decision interval) = 4
(this determines how many StDev from the mean the Cusum Limits are set)

NOTE: Using h=4 or h=5 and k=.5 will generally provide a cusum that has good ARL properties (per Montgomery SPC 4th ed.)

Compare a Cusum Chart with an EWMA Chart

Cusum Chart

cusum chart compared to ewma chart

EWMA Chart

ewma chart excel

Compare a Cusum Chart with an Individual Moving Range Chart

Cusum Chart

cusum chart compared to individuals chart

Individual Moving Range Chart

individuals chart in excel

Notice how the Cusum chart more quickly and clearly identifies the process shift.

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