In a Gage R&R study, the number of distinct categories represents the number of data groups that a measurement system is able to distinguish in the study. If the number of distinct categories is too small (less than 51), then the measurement system may not have the resolution needed to make meaningful distinctions between parts.
The formula for distinct categories is1,
ndc = 1.41(PV/GRR), where
PV is the part variation from the study and GRR is the calculated Gage R&R result.
There are two common solutions for addressing a low distinct categories value -
(1) Ensure that the parts selected for the Gage R&R study represent the full spectrum of variation observed in the process.
(2) Find a gage with at least 10x resolution versus the feature tolerance. For example, a diameter with a 0.1″ tolerance should be measured with a gage that has measurement increments of 0.01″ or less.
1 See AIAG, Measurement System Analysis, Third Edition, Page 117