On the significance of the excess number of strong MgII absorbers
  observed towards Gamma-ray bursts
  The number of strong (equivalent width > 1A) MgII absorbers observed towards
Gamma-ray bursts (GRBs) has been found to be statistically larger than the
number of strong absorbers towards quasi-stellar objects (QSOs). We formalize
this "MgII problem" and present a detailed explanation of the statistical tools
required to assess the significance of the discrepancy. We find the problem
exists at the 4{\sigma} level for GRBs with high-resolution spectra. It has
been suggested that the discrepancy can be resolved by the combination of a
dust obscuration bias towards QSOs, and a strong gravitational lensing bias
towards GRBs. We investigate one of the two most probable lensed GRBs that we
presented in our previous work (GRB020405; Rapoport et al.) and find it not to
be strongly gravitationally lensed, constraining the percentage of lensed GRBs
to be < 35% (2{\sigma}). Dust obscuration of QSOs has been estimated to be a
significant effect with dusty MgII systems removing ~20% of absorbed objects
from flux-limited QSO samples. We find that if ~30% of the strong MgII systems
towards QSOs are missing from the observed samples, then GRBs and QSOs would
have comparable numbers of absorbers per unit redshift. Thus, gravitational
lensing bias is likely to make only a modest contribution to solving the MgII
problem. However, if the dust obscuration bias has been slightly
underestimated, the MgII problem would no longer persist.
