  Three popular algorithms (FOF, DENMAX, and SKID) to identify halos in
cosmological N-body simulations are compared with each other and with the
predicted mass function from Press-Schechter theory. It is shown that the
resulting distribution of halo masses strongly depends upon the choice of free
parameters in the three algorithms, and therefore much care in their choice is
needed. For many parameter values, DENMAX and SKID have the tendency to include
in the halos particles at large distances from the halo center with low
peculiar velocities. FOF does not suffer from this problem, and its mass
distribution furthermore is reproduced well by the prediction from
Press-Schechter theory.
