Richard Clark wrote:
...what you would deem to be your best accuracy compared
to an absolute standard, or to a relative standard (instrumentation,
not computational).
______________

You weren't asking me, but still you may be interested in the link
below which leads to a good presentation of this by the NIST. A table
on Page 3 there shows a measurement uncertainty at the NIST test
facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency
range.

Field intensity measurements made using uncontrolled path conditions
are more a measure of the propagation environment and the pattern/
location of the receive antenna than they are of the absolute
performance of the transmitting antenna system. Such measurement
errors can be gross, and difficult to quantify.

http://ts.nist.gov/MeasurementServic...d/im-34-4b.pdf

RF