Representative examples of NDE clusters showing artefacts introduced by the subtraction of discrete sources near the cluster centre. The radio contours are drawn at ±2σ and spaced by factors of 2. In all the panels, the dashed cyan circle indicates the cluster centre and has a fixed diameter of 2′. Left: NDE cluster excluded from our analysis due to severe (positive and/or negative) artefacts. Middle: NDE cluster with SQ = 1 due to the absence of artefacts. Right: NDE cluster with SQ = 2 due to the presence of moderate residuals.