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Gage R&R, which stands for gage repeatability and reproducibility, is a statistical tool that measures the amount of variation in the measurement system arising from the measurement device and the people taking the measurement. See Gage R&R tools.
There’s an excellent post on the discussion forum by Stephen Curtis describing Gage R&R:
“Gage R&R is intended to be a study to measure the measurement error in measurement systems.”
Characterizing the Measurement Process
AIAG Automotive Industry Action Group publishes a booklet called Measurement Systems Analysis which answer your questions. Their website is http://www.aiag.org.
When measuring the product of any process, there are two sources of variation: the variation of the process itself and the variation of the measurement system. The purpose of conducting the GR&R is to be able to distinguish the former from the latter, and to reduce the measurement system variation if it is excessive.
Typically, a gage R&R is performed prior to using it. We repeat the gage R&R anytime we have a new operator or inspector, it is part of our training and certification process. We also repeat it annually to make sure we aren’t experiencing any erosion of skills. It is used as part of the Six Sigma DMAIC process for any variation project.