  Astrometric and radial-velocity planet detections track very similar motions,
and one generally expects that the statistical properties of the detections
would also be similar after they are scaled to the signal-to-noise ratio of the
underlying observations. I show that this expectation is realized for periods
small compared to the duration of the experiment P/T << 1, but not when P/T >~
1. At longer periods, the fact that models of astrometric observations must
take account of an extra nuisance parameter causes the mass error to begin
deteriorating at P/T ~ 0.8, as compared to P/T ~ 1.0 for RV. Moreover, the
deterioration is much less graceful. This qualitative difference carries over
to the more complicated case in which the planet is being monitored in the
presence of a distant companion that generates an approximately uniform
acceleration. The period errors begin deteriorating somewhat earlier in all
cases, but the situation is qualitatively similar to that of the mass errors.
These results imply that to preserve astrometric discovery space at the longest
accessible orbits (which nominally have the lowest-mass sensitivity) requires
supplementary observations to identify or rule out distant companions that
could contribute quasi-uniform acceleration.
