where 0 . 2 / 1 . 5 is the ratio of the intervals r > 0 . 8 and − 1 ≤ r ≤ 0 . 5. The fraction of the identified variables that are false positives is then F/N$_{r>}$$_{0}$$_{.}$$_{8}$ . 

Our ability to characterize sources also depends on the magnitudes of the object, because this determines how well we can measure the mid-IR colors. We divide sources into three photometry groups based on their mid-IR magnitudes. 

• Photometry Group 1 consists of all objects that after masking have three or four epochs of data, with no constraints on either their magnitudes or magnitude uncertainties. There are 474,179 Photometry Group 1 objects after masking, of which 5107, 2208, and 1071 meet the Variability Level 2, 3, and 4 criteria. 

• Photometry Group 2 is the subset of the Photometry Group 1 sources for which we can measure the [3.6] − [4.5] color. These sources must satisfy [3 . 6] < 19 . 7 mag and [4 . 5] < 19 . 3 mag with uncertainties in these bands smaller than 0.1 mag. There are 213,594 Photometry Group 2 objects, of which 1557, 668, and 359 meet the Variability Level 2, 3, and 4 criteria. 

• Photometry Group 3 is the subset of Photometry Group 2 objects for which we can also measure the [5.8] − [8.0] color. These sources are required to have uncertainties smaller than < 0 . 2 mag for the 5.8 and 8.0 µ m bands. There are 39,522 Photometry Group 3 objects, of which 775, 428, and 260 meet the Variability Level 2, 3, and 4 criteria. 

Table 1 provides these significance statistics for the SDWFS sources. Note, however, that our catalogs are limited to the objects detected on the template image – any transient which does not produce a detectable source in the template image, which is just the average of four epochs, will not be identified. The most variable, highly correlated ( r > 0 . 8) sources in the field are AGNs, peaking at σ$_{12}$ ≈ 20. 

We will make extensive use of the [3.6] − [4.5] versus [5.8] − [8.0] mid-IR color-color dis- tribution to characterize the sources. Figure 4 shows the distribution of all Photometry Group 3 SDWFS sources in this space. Eisenhardt et al. (2004) suggested that for bright sources the red plume in [3.6] − [4.5] color was likely to be dominated by AGNs. This was confirmed by Stern et al. (2005) based on the first year of spectroscopic observations by the AGES survey. Stern et al. (2005) defined a selection region, the “AGN wedge”, for mid-IR AGN selection that is also shown in Figure 4. For these Vega colors, stars lie at color 0 unless they have dusty winds or disks to produce a mid-IR excess. Using the spectral energy distribution (SED) models of Assef et al. (2010b), which are derived by fitting the SEDs of Bo¨ otes sources, we can illustrate the locations of various extragalactic populations as a 

