Our accuracy in the redshift measurements. Top panel: histogram of the distribution of velocity differences Δv = c × Δz/ (1 + z) for the set of 237 galaxies from the WPS_spec that were observed twice and whose redshift was independently measured. Such histogram is well fitted by a Gaussian of width σ = 132 km s^-1. Lower panel: redshift differences; the dashed (dotted) line corresponds to ±1(2)σ redshift difference.