Left column: probability of the injected periodic signal at P_c = 8.962 days existing in synthetic RV time-series, as a function of the injected semi-amplitude K_c with time-sampling identical to the published HARPS WF (C17a), the published CARMENES WF (S18), or their joint WF. Three sets of synthetic RV time-series are considered and contain K2-18c only (top row), K2-18b and c (middle row), or both planets plus a GP correlated noise model of stellar activity(bottom row). The shaded vertical region highlights the MAP and 1σ measured value of K_c = 4.63 ± 0.72 m s⁻¹ from C17a. Right column: probability of the injected periodic signal at P_c existing in synthetic RV time-series, with fixed K_c = 4.63 m s⁻¹, as a function of the number of RV measurements.