Distribution of optical extinction, parameterized by the color excess E(B − V), for the Primary Keck AO Merger Sample. Extinction values are drawn from the MPA-JHU DR8 Catalog (G. Kauffmann et al. 2003a; J. Brinchmann et al. 2004; C. A. Tremonti et al. 2004). The gray histogram shows the full sample, while the blue hatched bins and crimson hatched bins indicate systems without and with detected NIR substructure in the Keck AO imaging, respectively (see Section 4.1). This figure characterizes the extinction properties of the offset-selected merger sample and shows where systems with and without detected substructure reside in extinction space. The implications of these distributions for substructure detection and their relationship to SDSS/WISE positional offsets are discussed in Section 4.1.