Δ(N_H, los) as a function of Δ(t). Top: borus02-obtained values of Δ(N_H, los) between all observation pairs for each source, as a function of the time difference between said observations. Bottom: fractional difference in N_H, los between all observation pairs for each single source, with respect to the minimum N_H, los of the two, as a function of the time difference between said observations. The 25%, 50% and 75% quartiles of the distribution (Q1, Q2, Q3, respectively) are also shown as dashed horizontal lines.