  We investigate the correspondence between the variability seen in the stellar
and supermassive black holes. Galactic Black Hole (GBH) power density spectra
(PDS) are generally complex, and dependent on spectral state. In the low/hard
state the high-frequency rollover in the PDS moves in a way which is not simply
related to luminosity. Hence this feature can only be used as an approximate
indicator rather than as an accurate tracer of black hole mass in AGN. The
X-ray spectrum in the high/soft state is dominated by the disc in GBH, which is
rather stable. We show that the PDS of the Comptonized tail in GBH can be much
more variable, and that it is this which should be compared to AGN due to their
much lower disc temperature. This bandpass effect removes a problem in
interpreting the (often highly variable) Narrow Line Seyfert 1 (NLS1) galaxies
as the counterparts of the high mass accretion rate GBH. Additionally, we
speculate that some NLS1 (e.g. Akn 564) are counterparts of the very high
state. The Comptonized tail in this state is also highly variable, but with PDS
which can be roughly described as band-limited noise. This shape is similar to
that seen in the low/hard state, so merely seeing such band-limited noise in
the power spectrum of an AGN does not necessarily imply low luminosity. We also
stress that Cyg X-1, often used for comparison with AGN, is not a typical black
hole system due to its persistent nature. In particular, the shape of its power
spectrum in the high/soft state is markedly different from that of other
(transient) GBH systems in this state. The fact that the NLS1s NGC 4051 and
MCG-6-30-15 do appear to show a power spectrum similar to that of the high/soft
state of Cyg X-1 may lend observational support to theoretical speculation that
the Hydrogen ionization disc instability does not operate in AGN.
