Left: observational selection functions applied to the models. (a) Distance-dependent detection probability favouring stars on the near side of the disc (d < 8.2 kpc). (b) Midplane extinction reducing completeness to 30% at b = 0°. (c) Asymmetric longitude extinction with steeper decline at positive longitudes. The total probability is the product P_total(d, ℓ, b) = P(d) × P(b) × P(ℓ). Right: impact of selection on the NSD model’s distance (top), latitude (middle), and longitude (bottom) distributions, showing the unbiased sample (black) versus the observed sample after applying selection effects (red).