  Aims. Both the black hole mass and the X-ray luminosity of AGNs have been
found to be anti-correlated with the normalized excess variance ($\sigma_{\rm
rms}^2 $) of the X-ray light curves. We investigate which correlation with
$\sigma_{\rm rms}^2 $ is the intrinsic one.
  Methods. We divide a full sample of 33 AGNs (O' Neill et al. 2005) into two
sub-samples. The black hole masses of 17 objects in sub-sample 1 were
determined by the reverberation mapping or the stellar velocity dispersion. The
black hole masses of the remaining 16 objects were estimated from the
relationship between broad line region radius and optical luminosity
(sub-sample 2). Then partial correlation analysis, ordinary least squares
regression and K-S tests are performed on the full sample and the sub-samples,
respectively.
  Results. We find that $\sigma_{\rm rms}^2 $ seems to be intrinsically
correlated with the black hole mass in the full sample. However, this seems to
be caused by including the sub-sample 2 into the analysis, which introduces an
extra correlation between the black hole mass and the luminosity and
strengthens any correlation with the black hole mass artificially. Therefore,
the results from the full sample may be misleading. The results from the
sub-sample 1 show that the correlation between $\sigma_{\rm rms}^2 $ and the
X-ray luminosity may be the intrinsic one and therefore the anti-correlation
between $\sigma_{\rm rms}^2 $ and the black hole mass is doubtful.
