– 18 – are highly incomplete. This incompleteness is caused by the typical variability amplitude being comparable to the survey photometric errors, and is not due to the small number of available epochs. For our standard criteria, 14% of X-ray sources, 17% of 24 µ m selected AGN candidates, and 15% of spectroscopically confirmed AGNs are detected by their variability. The variable AGNs primarily occupy the mid-IR AGN selection wedge of Stern et al. (2005), with an extension to bluer [3 . 6] − [4 . 5] colors where the host galaxy dominates the mid-IR colors of low-luminosity AGNs (see Gorjian et al. 2008; Assef et al. 2010b). If we redefine the original Stern et al. (2005) selection wedge to encompass X-ray or variability- selected AGNs, we find that variable objects bluer than [3 . 6] − [4 . 5] > 0 . 3 mag are likely to be AGNs. This opens a new window for the Spitzer Warm Mission, where the [3 . 6] − [4 . 5] > 0 . 3 mag color cut combined with the variability criterion will be a solid indication of AGN activity. The structure function of z > 1 AGNs is well described by a power law with a logarithmic slope of γ = 0 . 56 ± 0 . 18 and amplitude S$_{0}$ = 0 . 11 ± 0 . 02 mag in [3.6] and γ = 0 . 44 ± 0 . 14, S$_{0}$ = 0 . 12 ± 0 . 01 mag in [4.5] for a rest-frame time lag of τ$_{0}$ = 2 years. These structure functions are steeper than those observed in optical bands, and we argue that this is due to the inability of either disk or dust emission to respond on the short timescales seen in optical studies. The mid-IR structure functions go approximately as τ 1 / $^{2}$, which corresponds to the short timescale ( τ ≪ τ$_{model}$ ) behavior in the damped random walk model used by Kelly et al. (2009), KozGLYPH<suppress>lowski et al. (2010), and MacLeod et al. (2010). If we examine the variability of various subsamples, we detect more variability in low luminosity objects observed at shorter rest-frame wavelengths. The mid-IR sources corresponding to X-ray and 24 µ m AGN have structure functions identical to that of z > 1 AGNs. However, this is not the case for the counterparts of radio sources which have S$_{0}$ ≃ 0 . 05 ± 0 . 01 mag in both [3.6] and [4.5] bands, as compared to S$_{0}$ ≃ 0 . 10 ± 0 . 01 for either z > 1 objects, X-ray, or MIPS sources. One explanation may be that the radio sample has more contamination by non-AGN (e.g., due to confusion between radio cores and lobes), another is that the radio-selected AGNs are accreting in a different mode (lower accretion rates or efficiencies; e.g., Tasse et al. 2008) and that variability in such a mode is inherently lower. Expanding the SDWFS survey with new epochs during the Spitzer Warm Mission would represent a significant improvement. Given our structure functions, we can estimate the results from expanding the SDWFS survey with new epochs. We considered two cases of (1) doing one new epoch in 2011 March and (2) doing new epochs in both 2011 and 2012 March. The improvements in the mean χ 2 are significant, with 〈 χ $^{2}$〉 = 4+3 . 0 S 2 $_{0}$/σ 2 and 5+4 . 3 S 2 $_{0}$/σ$^{2}$, respectively, for AGNs, compared to 〈 χ $^{2}$〉 = 4 or 5 for non-variable objects. These represent a significant improvement over the SDWFS baseline, where 〈 χ $^{2}$〉 = 3+1 . 6 S 2 $_{0}$/σ 2 for AGN and