– 13 – 5. Mid-IR Structure Functions Given the depth of the AGES redshift survey, all z > 1 objects are spectroscopically confirmed AGNs (see Figure 2 in Stern et al. 2005). We measured the mid-IR structure function of z > 1 Photometry Group 3 AGNs to fixed magnitude limits of [3 . 6] < 16, 17, and 18 mag. Only ∼ 10% of these objects were actually detected as significantly variable sources (see Table 5), so most of the observed variance in the magnitudes is at the noise level. This is a rather different regime from most optical studies, where the variance is dominated by variability, and makes it critically important to correctly characterize the contribution to the variance from the measurement errors. We do this by matching each AGN with four unresolved galaxies of nearly identical observed flux. We chose these galaxies to be Photometry Group 2 and 3 objects with colors [3 . 6] − [4 . 5] < 0 . 1 mag and 0.5 mag The structure function is a measure of the expected changes in an object’s flux as a function of the time between the measurements. The time lag τ is defined as the time difference between any two data points in a light curve, where the observed time lag is τ = | t$_{i}$ − t$_{j}$ | and the rest-frame time lag is τ = | t$_{i}$ − t$_{j}$ | / (1 + z ). For each time lag, one wants to find all pairs of points in a light curve and calculate the variance of the magnitude differences, Δ m = m$_{i}$ − m$_{j}$ . Such a variance can also be calculated by averaging over many objects of the same class (i.e., AGNs) for each time lag between any two epochs, as we do in this paper. In summary, small relatively shallow mid-IR variability surveys like SDWFS are a work- able but not a highly efficient means of identifying AGNs. Roughly speaking, the efficiencies are of order 10% for variability selection criterion providing relatively low false positive rates. It does, however, overcome the weaknesses of mid-IR color selection to identify AGNs outside the Stern et al. (2005) selection region. With difference imaging, the host galaxy flux that moves the source outside this color region ceases to be a problem other than through its effects on the photometric noise. AGNs are chosen to be in the IRAC AGN wedge, to show X-ray or radio emission, or to be optical point sources with non-stellar 24 µ m fluxes. The success rate for the spectroscopic identification of candidate AGNs in AGES is quite high. The sample of stars with spectra is very heterogeneous. Roughly half of the stars are F and G stars observed by AGES as spectrophotometric calibrators. Most of the remainder are K and M stars which were flagged as candidate AGNs either because of their X-ray fluxes or the presence of a mid-IR excess. We again see that the fraction of variable sources is highest for the AGNs, with roughly 11% meeting the Level 2 variability criterion.