  Using simulations we demonstrate that some of the published redshift
distributions of submillimetre galaxies (SMGs) at different wavelengths, which
were previously reported to be statistically different, are consistent with a
parent distribution of the same population of galaxies. The redshift
distributions which peak at z_med=2.9, 2.6, 2.2, 2.2, and 2.0 for galaxies
selected at 2 and 1.1 mm, and 870, 850, and 450 um respectively, can be derived
from a single parent redshift distribution, in contrast with previous studies.
The differences can be explained through wavelength selection, depth of the
surveys, and to a lesser degree, angular resolution. The main differences are
attributed to the temperature of the spectral energy distributions, as
shorter-wavelength maps select a hotter population of galaxies. Using the same
parent distribution and taking into account lensing bias we can also reproduce
the redshift distribution of 1.4 mm-selected ultra-bright galaxies, which peaks
at z_med=3.4. However, the redshift distribution of 450 um-selected galaxies in
the deepest surveys, which peaks at z_med=1.4, cannot be reproduced from the
same parent population with just these selection effects. In order to explain
this distribution we have to add another population of galaxies or include
different selection biases.
