  Ninety gravitational wave events have been detected by the LIGO-Virgo-KAGRA
network and are released in the Gravitational-Wave Transient Catalog. Among
these events, 83 cases are definitely binary black hole mergers since the
masses of all the objects involved significantly exceed the upper limit of
neutron stars. The black holes in these merger events naturally form two
interesting samples, a pre-merger sample that includes all the black holes
before the mergers and a post-merger sample that consists of the black holes
generated during the merging processes. The former represents black holes that
once existed in the Universe, while the latter represents newly born black
holes. Here we present a statistical analysis on these two samples. The
non-parametric $\tau$ statistic method is adopted to correct for the
observational selection effect. The Lynden-Bell's $C^{-}$ method is further
applied to derive the mass distribution and density function of black holes. It
is found that the mass distribution can be expressed as a broken power-law
function. More interestingly, the power-law index in the high mass region is
comparable for the two samples. The number density of black holes is found to
depend on redshift as $\rho(z) \propto z^{-2.06}$-$z^{-2.12}$ based on the two
samples. Implications of these findings on the origin of black holes are
discussed.
