Line areas from Gaussian fits versus the true line area. Results are shown for C¹⁸O and ¹³CO (upper and lower frames, respectively) and for noise levels of σ_T = 1 % and σ_T = 10% of the peak value in each spectrum (left and right frames, respectively). The blue histograms contain all spectra with T^max > 10 mK and the red histograms the strongest spectra with T^max > 1 K. The unfilled histograms correspond to single-component fits and the filled histograms to one to three component fits, where the number of components was selected based on the AIC criterion.