Histograms for the ratio of the velocity dispersion recovered by single-component Gaussian fits (σ_v(fit)) and the true velocity dispersion of the synthetic spectra (σ_v(true)). Histograms correspond to spectrum subsets where the best model has only a single velocity component (AIC, based on the AIC criterion), spectra have only a single peak (“1-peak”), and the intersection of these two samples (“both”). The true velocity dispersion was calculated either for the whole spectrum (filled histograms) or only within 2.5σ_v(fit) around the fitted centre velocity (unfilled histograms). Results are shown for C¹⁸O and ¹³CO (upper and lower frames, respectively) and for noise levels of σ_T = 1% and σ_T = 10% (left and right frames, respectively).