Here are some of the frequently asked questions (FAQ) we have been asked. We will add to this over time…


Can someone share how they have done gage R&R studies for destructive testing?

The only approach I’m aware of is to take samples with as close to identical properties (manufactured in the same lot, right after each other, with no other difference in people or machines), in order to minimize the variation between the parts. If possible, take one part and separate it into smaller pieces to perform the testing. Then you would treat these parts as “repeats” in the Gage R&R, even though technically they are not repeats (already destroyed). The problem is that variation in repeatability will be combined with any variation in these “repeat” parts, so it’s not a perfect Gage R&R, but should give you a good idea whether the measurement system is adequate.

If you’d like to learn more about Gage R&R or measurement system analysis, download our Gage R&R Excel template >>>


I have a process with 10 samples per subgroup, and 20 subgroups total. When I only plot the average of the subgroups on an Individuals chart (ignoring the individual data points completely), the data values fall within the control limits. However, when I run the data in an X-bar and R chart (using the 10 samples per subgroup), the data points are out of control?

The control limits will be different. The Individuals chart calculates the standard deviation for the control limits based upon the overall moving range average (average difference from current data point to previous data point), divided by a constant, d2. In the X-bar and R charts, the control limits are calculated based on the variation within the subgroup, which is the overall range average (ranges of each subgroup), multiplied by A2.

Control Limits for X-bar and R chart

x-bar and R control chart formula

Control Limits for Individuals chart

Individuals x and moving range R control chart formula

If you’d like to learn more about control charts, download our Control Charts PowerPoint course >>>

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