Test of fitting N₂H⁺ observations containing two velocity components. The upper frames show simulated spectra (black lines) with 0.3 K observational noise and varying velocity difference δυ between the two components. Fits of a single velocity component are plotted in red for a number of noise realisations. The lower frame shows the recovered optical depth (blue line) and column density (red line) as a function of the velocity offset δυ. The shaded regions correspond to the inter-quartile range, which is computed from 128 noise realisations. The y-axis shows the ratio between the estimates and the true values.