Simulation of p_mas vs. N relation for POL-2 data. The figure shows the slopes of the fitted least squares lines as a function of the column density threshold used to select data. The black line shows the fits to the original observations. The red curve shows the median slope for simulations that assume an intrinsically constant polarisation fraction of p = 1.5% and the shaded areas correspond to [1%, 99%], [10%, 90%], and [16%, 84%] confidence intervals.