  Observational surveys of galaxies are not trivially related to single-epoch
snapshots from computer simulations. Observationally, an increase in the
distance along the line-of-sight corresponds to an earlier cosmic time at which
the properties of the surveyed galaxy population may change. The effect of
observing a survey volume along the light-cone must be considered in the regime
where the mass function of galaxies varies exponentially with redshift. This
occurs when the halos under consideration are rare, that is either when they
are very massive or observed at high-redshift. While the effect of the
light-cone is negligible for narrow-band surveys of Lyman-alpha emitters, it
can be significant for drop-out surveys of Lyman-break galaxies (LBGs) where
the selection functions of the photometric bands are broad. Since there are
exponentially more halos at the low-redshift end of the survey, the
low-redshift tail of the selection function contains a disproportionate
fraction of the galaxies observed in the survey. This leads to a redshift
probability distribution (RPD) for the dropout LBGs with a mean less than that
of the photometric selection function (PHSF) by an amount of order the standard
deviation of the PHSF. The inferred mass function of galaxies is then shallower
than the true mass function at a single redshift with the abundance at the
high-mass end being twice or more as large as expected. Moreover, the
statistical moments of the count of galaxies calculated ignoring the light-cone
effect, deviate from the actual values.
