Left panel: mean (bias) of S′_std(N_gal)/S_std(< R₂₀₀) as a function of the number of galaxies N_gal, calculated by choosing galaxies within 100% (solid black line), 1/2 (solid blue line), 1/3 (solid red line), and 1/4 (solid green line) of the complete cluster member samples. Right panel: variance of S′_std(N_gal)/S_std(< R₂₀₀) as a function of the number of galaxies N_gal. The dashed line represents the theoretical expectation for the variance of the dispersion according to Eq. (10).