– 30 – Fig. 9.— Rest-frame mid-IR AGN structure functions for the objects with [3 . 6] < 18 mag. We divide the objects into two redshift bins, 1 < z < 2 (filled symbols) and 2 < z < 3 (open symbols), and also into two brightness levels, objects brighter (circles) and fainter (squares) than the median for the redshift bin. For time lags of half a year and longer, fainter and lower redshift sources show greater variability. The solid lines are the mid-IR structure function fits from the bottom panel of Figure 8, and the dotted lines are the i -band rest-frame structure functions from Vanden Berk et al. (2004).