– 11 – 4. Results We will now characterize the variable sources using three approaches. First, we simply examine the distribution of objects in mid-IR color. Second, we examine the variability of AGN candidates identified at other wavelengths. Finally, we consider the spectral properties of the objects with known redshifts. Figure 5 and Table 3 characterize the distribution of objects in mid-IR color. The Photometry Group 3, Variability Level 2 candidates are concentrated in either the modified AGN wedge, with a distribution very similar to that of X-ray sources (Gorjian et al. 2008; see Figure 4), or in the stellar box. While only 0.5% of the Photometry Group 3 objects outside these two regions are Variability Level 2 sources, 7.4%, 4.6%, and 1.1% of the sources in the old AGN, modified AGN, and stellar regions are Variability Level 2 sources. The estimated false positive rates are 44%, 6%, 7%, and 58% for these four regions (outside, old AGN wedge, modified AGN wedge, stellar box) in the color-color space, respectively. This simply means that, for example, of the 7.4% variable sources in the old AGN wedge 6% are false positives, so in fact 7.4% × (1 − 0 . 06) = 7 . 0% of all objects are real variables and 7.4% × 0 . 06 = 0 . 4% of all objects are falsely identified as variable. Increasing the variability significance level reduces the variable fractions roughly in the same proportion, suggesting that even the Variability Level 2 candidates are dominated by true variables. The distribution also validates our assumption that the variability is dominated by AGN – 75% of the candidates lie in the modified AGN region for Variability Level 4. The selection remains pure even for Variability Level 2 because the requirement that the [3.6] and [4.5] light curves are tightly correlated ( r > 0 . 8) eliminates most of the contamination even when we only require σ$_{12}$ > 2 (see Figure 3). Most variable sources in the modified AGN wedge are known to be AGNs based on other evidence. Only 16%, 10%, and 7% of the Photometry Group 3, Variability Level 2, 3, and 4 sources lack photometric or spectroscopic signs of AGN activity. Most of these, however, are likely to be false positives given our estimated false positive rates of 7%, 4%, and 3%. For the Photometry Group 2 objects, where we lack accurate [5.8] − [8.0] color, we can only investigate the distribution in [3.6] − [4.5] color, and the side panel to Figure 5 shows the fraction of variable sources as a function of this color. Not surprisingly, the fraction is high ( ∼ 10%) when the color is red, corresponding to AGNs, and then drops dramatically for colors consistent with normal galaxies. The fraction of variables in the other two photometry groups are dramatically lower than for Photometry Group 3 for two reasons. First, a source must be intrinsically more variable in order to meet any variability criterion because the intrinsic measurement errors are larger. Second, with less well-measured colors, objects are scattered from their true location in the color-color space, and the differential scattering of galaxies into the AGN regions and AGNs out of these regions dilutes the selection criterion.