– 7 – In order to understand the nature of the variable sources, we matched the SDWFS sources to several of the available data sets for the Bo¨ otes field, focusing on information related to AGN activity. Thus, we matched to the X-ray and radio catalogs, to the subset of 24 µ m sources with high probabilities of being AGNs, and to the AGES spectroscopic data. We used the X-ray catalogs from the XBo¨ otes Chandra survey covering 9.3 deg 2 of the Bo¨ otes field in the 0.5-7 keV band (Murray et al. 2005; Kenter et al. 2005; Brand et al. 2006; Gorjian et al. 2008). We matched a version of the Murray et al. (2005) catalogs extended downward in flux from 3293 sources with 4 or more counts to 4642 sources with 2 or more counts. While using 2 count (about 4 × 10 − 15 erg cm − 2 s − $^{1}$) X-ray sources sounds peculiar, the backgrounds in XBo¨ otes are so low that almost all such sources with mid-IR identifications will be real, particularly if the match is to one of the relatively rare mid-IR variable sources. Using the Bayesian matching method of Brand et al. (2006), we identified 3363 X-ray sources with at least two epochs of SDWFS data. We used the same procedures to match to the 20 cm FIRST (Becker et al. 1995) and 1.4 GHz WSRT (de Vries et al. 2002) radio catalogs. Restricting the matching to likely radio point sources in the FIRST and WSRT surveys, we found 322 and 2118 matches, respectively, to SDWFS sources with at least two epochs of data. We know from AGES that most 24 µ m, optical point sources satisfying the criterion that I > 18 − 2 . 5 log( F$_{24}$/ mJy) are quasars (Brown et al. 2006; Eckart et al. 2010; Assef et al. 2010a), where the optical criterion is used to eliminate stars. Finally, we matched to the AGES redshift survey, finding 19 , 885 (18 , 288, 1387, 497) SDWFS sources (with redshifts z > 0 , 1 , 2). The modified SDSS pipeline used for the AGES redshifts (HSRED; Cool et al. 2008) provides an estimate of the best fit spectral template used for the fits that can be used to classify objects. More generally, we have all the galaxy and AGN target selection codes used by AGES to select and prioritize spectroscopic targets. 3. Variability For each light curve i (from Table 2) with at least three epochs in both the [3.6] and [4.5] bands, we computed the standard deviation v ([ X ]) i for that light curve as a function of (Vega) apparent magnitude m ([ X ]) $^{i}$, as shown in Figure 2. We also calculated the variability covariance C i 12 and Pearson’s correlation r i coefficients between the two channels, where