Number of distinct data categories and gage repeatability and reproducibility. A double (but single) requirement

Measurement systems analysis has been widely used in many industries, especially those related to the automotive sector as part of statistical process control practices. Over the years, new elements have been added to the initial requirements for system acceptance, centered on the values of repeatability and reproducibility (integrated in indexes as GRR and GRR%). One of these elements, the number of distinct data categories (NDC) has been treated as an independent requirement. In this paper, we show that NDC and GRR are not independent and that they are in fact both linked by an exact relationship. This therefore makes the double requirement redundant, confusing and unnecessary, except maybe to enhance some conceptual questions.