  We investigate the relationship between 5 GHz interstellar scintillation
(ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn
from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and
the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We
discover that the strongest scintillators at 5 GHz (modulation index, $m_5 \geq
0.02$) all exhibit strong 15 GHz intrinsic variability ($m_{15} \geq 0.1$).
This relationship can be attributed mainly to the mutual dependence of
intrinsic variability and ISS amplitudes on radio core compactness at $\sim
100\, \mu$as scales, and to a lesser extent, on their mutual dependences on
source flux density, arcsec-scale core dominance and redshift. However, not all
sources displaying strong intrinsic variations show high amplitude
scintillation, since ISS is also strongly dependent on Galactic line-of-sight
scattering properties. This observed relationship between intrinsic variability
and ISS highlights the importance of optimizing the observing frequency,
cadence, timespan and sky coverage of future radio variability surveys, such
that these two effects can be better distinguished to study the underlying
physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud
sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet
sources. This relationship is weaker than the known correlation between
gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to
jet opacity effects.
