Top panel: Merger fraction in real samples over the average merger fraction in random samples, F_m, as a function of the number density n₁. The dotted line marks identity. The dashed line marks the best least-squares linear fit to the data, F_m = 4.25 − 0.27n₁. Bottom panel: Cosmic variance in real samples over the average cosmic variance in random samples, Σ_v, as a function of the number density n₁. The dotted line marks identity. The dashed line is the error-weighted average of the data, Σ_v = 0.81 ± 0.04, and the coloured area its 68% confidence interval. A colour version of this plot is available in the electronic edition.