  A sample of 123 radio sources that exhibit significant variations at 1.4 GHz
on a seven year base-line has been created using FIRST VLA B-array data from
1995 and 2002 on a strip at dec=0 near the south Galactic cap. This sample
spans the range of radio flux densities from ~2 to 1000 mJy. It presents both
in size and radio flux density range a unique starting point for variability
studies of galaxies and quasars harboring lower luminosity Active Galactic
Nuclei (AGN). We find, by comparing our variable and non-variable control
samples to the Sloan Digital Sky Survey the following: 1) The quasar fraction
of both the variable and non-variable samples declines as a function of
declining radio flux density levels; 2) our variable sample contains a
consistently higher fraction of quasars than the non-variable control sample,
irrespective of radio flux; 3) the variable sources are almost twice as likely
to be retrieved from the optical SDSS data than the non-variable ones; 4) based
on relative numbers, we estimate that quasars are about five times more likely
to harbor a variable radio source than are galaxies; and 5) there does not
appear to be any significant optical color offset between the two samples, even
though the suggestive trend for sources to be bluer when variable has been
detected before and may be real. This leads us to conclude that both radio
variability and radio flux density levels, in combination with accurate optical
information, are important discriminators in the study of (radio) variability
of galaxies. The latter start to dominate the source counts below about 20 mJy.
In any case, variability appears to be an intrinsic property of radio sources,
and is not limited to quasars. Radio variability at low flux density levels may
offer a unique tool in AGN unification studies.
