Ask the Expert

Can control charts detect small process shifts?

Typical control charts, such as Xbar & R and X & Mr, indicate whether or not a process is statistically stable. They detect process changes greater than 1.5σ and, because of this, are not the best tool to detect small process shifts. To detect small shifts, defined as shifts less than 1σ, use the EWMA or Cusum charts.

ewma

The EWMA (Exponential Weighted Moving Average) chart weights moving average values according to time. The more recent the value, the greater the weight assigned to that value and, in effect, the greater the emphasis given to that value.

tabularcusum

The Cusum (Cumulative Sum) chart plots the cumulative sum of deviations from a target value. These charts detect shifts from the process mean as small as 0.5σ. Cusum charts are plotted in 2 forms – V-Mask and Tabular Cusum (available in WinSPC).

Although EWMA and Cusum charts are very valuable in monitoring continuous processes where small changes have significant effects, it is not recommended that they be used as process improvement tools. Rather, process improvement decisions should be made using other tools such as control, pre-control and Pareto charts.

[ Back ]

News & Publications

Have a question for our experts? Send an email to This email address is being protected from spam bots, you need Javascript enabled to view it or complete and submit the following form:

Signup & Receive a Free Copy of
"Top 10 Mistakes Made in
SPC Implementations"

Watch this on-demand Webinar to discover new ways to:

Monetarily measure the value of your SPC program
Uncover how much real-time SPC software will impact your bottom line
Win executive support for new or existing programs

watch it now...

Free technical white papers to make choosing a quality solution for your organization easier.