  Recent observational studies using the Hubble Space Telescope (HST) have
derived the center-of-mass proper motion (CMPM) of the Large Magellanic Cloud
(LMC). Although these studies carefully treated both rotation and perspective
effects in deriving the proper motion for each of the sampled fields, they did
not consider the effects of local random motion in the derivation. This means
that the average PM of the fields (i.e., the observed CMPM) could significantly
deviate from the true CMPM, because the effect of local random motion can not
be close to zero in making the average PM for the small number of the fields
(~10). We discuss how significantly the observationally derived CMPM can
deviate from the true CMPM by applying the same method as used in the
observations for a dynamical model of the LMC with a known true CMPM. We find
that the deviation can be as large as ~ 50 km/s (~0.21 mas/yr), if the LMC has
a thick disk and a maximum circular velocity of ~120$ km/s. We also find that
the deviation depends both on the total number of the sampled fields and on
structure and kinematics of the LMC. We therefore suggest that there is a
possibility that the observed CMPM of the LMC deviates from the true one to
some extent. We also show that a simple mean of PM for a large number of the
LMC fields (~1000) can be much closer to the true CMPM.
