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We report on the variability of 443 flat spectrum, compact radio sourcesmonitored using the VLA for 3 days in 4 epochs at ~ 4 month intervals at 5 GHzas part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV)survey. Over half of these sources exhibited 2-10% rms variations on timescalesover 2 days. We analyzed the variations by two independent methods, and findthat the rms variability amplitudes of the sources correlate with the emissionmeasure in the ionized Interstellar Medium along their respective lines ofsight. We thus link the variations with interstellar scintillation ofcomponents of these sources, with some (unknown) fraction of the total fluxdensity contained within a compact region of angular diameter in the range10-50 micro-arcseconds. We also find that the variations decrease for high meanflux density sources and, most importantly, for high redshift sources. Thedecrease in variability is probably due either to an increase in the apparentdiameter of the source, or a decrease in the flux density of the compactfraction beyond z ~ 2. Here we present a statistical analysis of these results,and a future paper will the discuss the cosmological implications in detail.

The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs