black hole mass, but remains constant with redshift and luminosity. 

Far less is known about the near-IR and mid-IR variability of quasars. Since AGN lumi- nosities do vary in time at optical wavelengths, we expect to find some variability in the IR, but it could be heavily smoothed by averaging the response over the large scales of either the cooler parts of the disk or the absorbing dust. Glass (2004) found that the majority (39/41) of the surveyed low-luminosity nearby Seyferts do vary in the near-IR and mid-IR, and that the variability is most apparent at longer wavelengths due to diminishing flux from the host galaxy. The most extensive work to date on near-IR and mid-IR variability of quasars is that of Neugebauer & Matthews (1999). These authors studied 25 low-redshift quasars ( z ≃ 0 . 1), which were observed in five bands (1.3 – 10.6 µ m) for ∼ 30 years. The amplitude of variations for radio loud (quiet) quasars is ∼ 0 . 3 mag (0.1 mag) on rest-frame time lags (time differ- ences between any two epochs) of 2 years at 10.6 µ m. For one quasar, PG 1226+023, they measured structure functions in five bands, all showing ∼ 0 . 1 mag variations for a rest-frame time lag of 2 years. Another example is the work of Suganuma et al. (2006), who found a delayed response between the K - and V -band variability in four Seyfert 1 galaxies. These authors showed that K -band light curves have a fairly tight time lag–luminosity relation, τ ≈ L 1 / $^{2}$, while broad line region (BLR) time lags are more scattered. The interpretation is that the K -band emission is mainly coming from dusty clouds bounding the BLR, and that the size scale of these dust clouds is very dependent on the AGN luminosity, since they have to be colder than the dust sublimation temperature. 

In this paper, for the first time, we investigate the variability of a truly large num- ber of mid-IR sources, using the four-epoch Spitzer Deep Wide Field Survey (SDWFS; Ashby et al. 2009) of the Bo¨ otes field of the NOAO Deep Wide Field Survey (NDWFS; Jannuzi & Dey 1999). The two goals are to characterize the mid-IR variability of a large sample of AGNs, and to use the variability to identify additional AGNs. While mid-IR quasar selection (Stern et al. 2005) works very well when the AGN dominates the luminos- ity (even for quasars behind dense stellar fields such as LMC/SMC; KozGLYPH<suppress>lowski & Kochanek 2009), the method begins to fail as the luminosity becomes comparable to that of the host (Gorjian et al. 2008; Assef et al. 2010b). With mid-IR variability, we should be able to iden- tify such sources and extend the sample of mid-IR selected quasars to lower luminosities. We also make use of the extensive wavelength coverage of the field, including X-ray (XBo¨ otes; Murray et al. 2005), the earlier Infrared Array Camera (IRAC) Shallow Survey that be- came the first epoch of SDWFS (Eisenhardt et al. 2004), the 24 µ m survey of the field (see Houck et al. 2005), and the FIRST (Becker et al. 1995) and WSRT (de Vries et al. 2002) radio surveys. Based on these data, the AGN and Galaxy Evolution Survey (AGES; C. S. Kochanek et al. 2010, in preparation) selected galaxies and AGNs at all these wavelengths for spectroscopic follow up using the Hectospec instrument on the MMT (Fabricant et al. 

