DM difference between measured and predicted, as a function of the NE2001 predicted DM. Associated pulsars are given in diamonds, where open, black diamonds are SNR associations, filled orange diamonds represent PWN associations and black lined diamonds with filled centres are plerionic systems. The pulsars unassociated with a SNR or PWN are shown in turquoise. The offset DM for the associated sample is represented by the grey, solid line with its one σ error-region shaded. The fit, and the one σ error-region, to the offset for the unassociated sample is shown as the grey dashed line and overlaps with the black solid x-axis.