  The various experimental data at AGS, SPS and RHIC energies on hadron
particle yields for central heavy ion collisions are investigated by employing
a generalized statistical density operator, that allows for a well-defined
anisotropic local momentum distribution for each particle species, specified by
a common streaming velocity parameter. The individual particle ratios are
rather insensitive to a change in this new intensive parameter. This leads to
the conclusion that the reproduction of particle ratios by a statistical
treatment does not imply the existence of a fully isotropic local momentum
distribution at hadrochemical freeze-out, i.e. a state of almost complete
thermal equilibrium.
