Many systematic studies have been carried out in the last two decades to assess the accuracy of
the QHR. There is now overwhelming experimental evidence that the QHR is a universal quantity.
It is independent of host material, device and plateau number at the level of a few parts in 1010
which is the resolution of today’s measurement techniques. As a consequence, the QHR is used
by all the major national metrology institutes as a dc standard for resistance. The reproducibility
of this quantum standard is two orders of magnitude better than the absolute realization of the
ohm in the SI. By fixing conventional values for the von Klitzing constant RK and the Josephson
constant KJ, the worldwide consistency of the electrical measurements has improved considerably