There is nearly a factor of four difference in the number density of intervening MgII absorbers as determined from gamma-ray burst ( GRB ) and quasar lines of sight .
We use a Monte-Carlo simulation to test if a dust extinction bias can account for this discrepancy .
We apply an empirically determined relationship between dust column density and MgII rest equivalent width to simulated quasar sight-lines and model the underlying number of quasars that must be present to explain the published magnitude distribution of SDSS quasars .
We find that an input MgII number density dn / dz of 0.273 \pm 0.002 over the range 0.4 \leq z \leq 2.0 and with MgII equivalent width W _ { 0 } \geq 1.0 \mathrm { \AA } accurately reproduces observed distributions .
From this value , we conclude that a dust obstruction bias can not be the sole cause of the observed discrepancy between GRB and quasar sight-lines : this bias is likely to reduce the discrepancy only by \sim 10 \% .