Dressler-Shectman (DS) plot. It shows the x, y location of the spectroscopically-confirmed proto-cluster members, centered on 7C 1756+6520. The DS test is applied to proto-cluster members with z within the range [−3σ_cl, +3σ_cl] from z_cl. The radii of the plotted circles are proportional to e^δ/2, where δ is the DS measurement of local deviation from the global velocity dispersion and mean recession velocity (cf. Eq. (2)). Depending on in which bin falls, the circles are: blue and dashed [−3σ_cl, −1σ_cl], green [−1σ_cl, +1σ_cl], red and hashed [+1σ_cl, +3σ_cl]. The black and dotted circle is the galaxy MSC2, excluded from the DS test because it has a that does not fall in the range [−3σ_cl, +3σ_cl]. The number of proto-cluster members N_g, the statistics Δ, and the statistical significance P are also given on the figure.