The mystery of the missing GRB redshifts
  It is clear that optical selection effects have distorted the "true" GRB
redshift distribution to its presently observed biased distribution. We
constrain a statistically optimal model that implies GRB host galaxy dust
extinction could account for up to 40% of missing optical afterglows and
redshifts in $z = 0-3$, but the bias is negligible at very high-$z$. The
limiting sensitivity of the telescopes, and the time to acquire
spectroscopic/photometric redshifts, are significant sources of bias for the
very high-$z$ sample. We caution on constraining star formation rate and
luminosity evolution using the GRB redshift distribution without accounting for
these selection effects.
