Top: number of positive (red) and negative (blue) detections as a function of S/N, which is defined as the ratio of the peak emission to the noise in the flux map. The dotted black line shows the distribution of positive (or negative) detections obtained from 100 pure noise datacubes and normalised to one datacube. Bottom: number of positive (red) and negative (blue) detections as a function of “spectral” significance, i.e. relative to the noise in the same spectrum, which is defined as the ratio of the flux line to the relative error.