  The idea of a unified description of supermassive and stellar black holes has
been supported by the extension of the empirical radio/X-ray correlation from
X-ray binaries to active galactic nuclei through the inclusion of a mass term.
This has lead to the so-called fundamental plane of black hole activity in the
black hole mass, radio and X-ray luminosity space. Two incarnations of this
fundamental plane have so far been suggested using different underlying models
and using two different samples of accreting black holes. We present revised
samples for both studies together with a refined statistical analysis using
measured errors of the observables. This method is used to compare the two
samples, discuss selection effects, and infer parameters for the fundamental
plane in a homogeneous way. We show that strongly sub-Eddington objects in a
state equivalent to the low/hard state of X-ray binaries follow the fundamental
plane very tightly; the scatter is comparable to the measurement errors.
However, we find that the estimated parameters depend strongly on the
assumptions made on the sources of scatter and the relative weight of the
different AGN classes in the sample. Using only hard state objects, the
fundamental plane is in agreement with the prediction of a simple uncooled
synchrotron/jet model for the emitted radiation. Inclusion of high-state
objects increases the scatter and moves the correlation closer to a disk/jet
model. This is qualitatively consistent with a picture where low-state objects
are largely dominated by jet emission while high-state objects have a strong
contribution from an accretion disk.
