Effect of a partial coverage of quasar broad-line regions by intervening
  H$_2$-bearing clouds
  We consider the effect of a partial coverage of quasar broad-line regions
(QSO BLRs) by intervening H$_2$-bearing clouds when a part of quasar (QSO)
radiation passes by a cloud not taking part in formation of an absorption-line
system in the QSO spectrum. That leads to modification of observable absorption
line profiles and consequently to a bias in physical parameters derived from
standard absorption line analysis. In application to the H$_2$ {absorption}
systems the effect has been revealed in the analysis of H$_2$ absorption system
in the spectrum of Q~1232+082 (Ivanchik et al. 2010, Balashev et al. 2011). We
estimate a probability of the effect to be detected in QSO spectra. To do this
we derive distribution of BLR sizes of high-z QSOs from Sloan Digital Sky
Survey (SDSS) Data Release 9 (DR9) catalogue and assume different distributions
of cloud sizes. We conclude that the low limit of the probability is about
$11\%$. The latest researches shows that about a fifth of observed H$_2$
absorption systems can be partially covered. Accounting of the effect may allow
to revise significantly physical parameters of interstellar clouds obtained by
the spectral analysis.
