  The two most common mass estimators that use velocity dispersions and
positions of objects in a stellar system, the virial mass estimator (VME) and
the projected mass estimator (PME), are revisited and tested using N-body
experiments. We consider here only spherical, isolated, and isotropic velocity
dispersion systems. We have found that the PME can overestimate masses by ~
20%, for realistic cluster mass profiles, if applied only to regions around the
total system's "effective" radius. The VME can yield a correct mass at
different radii provided that the total potential energy term is correctly
taken into account and the system is completely sampled; otherwise, it may lead
to similar errors as the PME. A surface pressure (3PV) term recently alluded to
be usually neglected when using the VME and therefore required as a correction
term is here found not necessarily required, although it can be used to yield a
reasonable correction term. The preferred method here, however, is the virial
theorem due to its simplicity and better agreement with N-body experiments. The
possible reasons for the mass discrepancies found when using the PME and the
VME in some N-body simulations are also briefly discussed.
