  Although a source star is fainter than the detection limit imposed by
crowding, it is still possible to detect an event if the star is located in the
seeing disk of a bright star is and gravitationally amplified: amplification
bias. Using a well-constrained luminosity function, I show that $\sim 40\%$ of
events detected toward the Galactic bulge are affected by amplification bias
and the optical depth might be overestimated by a factor $\sim 1.7$. In
addition, I show that if one takes amplification bias into consideration, the
observed time scale distribution matches significantly better, especially in
the short time-scale region, with the distribution expected from a
mass-spectrum model in which lenses are composed of the known stellar
population plus an additional population of brown dwarfs than it is without the
effect of the amplification bias.
