Probability distribution functions (P(DM_IGM)) for FRBs of source redshift z_s = 1 accounting for DM_IGM sightline variance due to collapsed systems. The Gaussian approximation assumed in this work is compared to more complex McQuinn (2014) baryonic halo models (tracing top hat functions, NFW profiles, and cosmological simulations, respectively, obtained via. private communication). Profiles are normalised to ∫ P(DM |z_s = 1) d DM = 1000.