  We review five studies of the gamma-ray burst (GRB) luminosity function that
are based on one or more of the proposed luminosity criteria, i.e.,
variability, spectral lag and spectral hardness. A comparison between the
resulting luminosity functions at redshift zero shows considerable divergence.
We derive for each of the luminosity functions the expected source counts and
the Euclidean value of <V/Vmax>. The source counts exhibit large differences
from the observed counts. In most cases the value of <V/Vmax> differs
substantially from the observed value of 0.34 for BATSE-based samples, with
offsets ranging from 6-24 sigmas. Given that the value of <V/Vmax> is well
established and that it is a cosmological distance indicator, we recommend that
any study of the GRB luminosity function take its value into account.
