Constraints on p and R_V when dividing data into regions with low (D^T < 3 MJy sr^-1) and high (D^T > 3 MJy sr^-1) dust column density. There is no evidence for any correlation between the dust column density and dust properties. Upper left: results for CCM dust for all regions with D^T < 3 MJy sr^-1 corresponding to 95.4% CL for one parameter of p = 0.020 ± 0.0016 and R_V = 3.06 ± 0.56. Upper right: results for CCM dust at D^T > 3 MJy sr^-1 giving p = 0.019 ± 0.0019 and R_V = 3.09 ± 0.65. Lower left: results for Fitzpatrick dust at D^T < 3 MJy sr^-1 giving p = 0.019 ± 0.0016 and R_V = 3.28 ± 0.37. Lower right: results for Fitzpatrick dust at D^T > 3 MJy sr^-1 giving p = 0.018 ± 0.0018 and R_V = 3.23 ± 0.41.