  Using very long baseline interferometry data for the sources that comprise
the third International Celestial Reference Frame (ICRF3), we examine the
quality of the formal source position uncertainties of ICRF3 by determining the
excess astrometric variability (unexplained variance) for each source as a
function of time. We also quantify multiple qualitatively distinct aspects of
astrometric variability seen in the data, using a variety of metrics. Average
position offsets, statistical dispersion measures, and coherent trends over
time as explored by smoothing the data are combined to characterize the most
and least positionally stable ICRF3 sources. We find a notable dependence of
the excess variance and statistical variability measures on declination, as is
expected for unmodeled ionospheric delay errors and the northern hemisphere
dominated network geometries of most astrometric and geodetic observing
campaigns.
