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 5^{1}), then the measurement system may not have the resolution needed to make meaningful distinctions between parts.

The formula for distinct categories is^{1},

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