Magnification Bias in Galactic Microlensing Searches
  It is shown that a significant amount of detectable gravitational microlensing events that could potentially be found by MAssively Parallel Photometry (MAPP) projects (such as the MACHO, EROS and OGLE collaborations) will occur for stars too dim to be easily noticed individually by these projects. This is the result of a large magnification bias effect, a bias of including high magnification events in any flux-limited sample. The probability of detecting these events may be as high as 2.3 times the lensing probability of stars currently being monitored by MAPP collaborations. 
Robert J. Nemiroff (George Mason University and NASA Goddard Space   Flight Center)