Emission ratio of H₂O 1₁₀–1₀₁/CO 3–2 as a function of absolute velocity. All spectra have been centred at 0 km s^-1. The top panel shows the ratio for all line wings where both H₂O and CO emission exceeds 4σ. Line wings from Class 0 sources are shown in red, wings from Class I sources in blue. The average value is shown in black. The bottom panel shows the average value with the standard deviation shown in grey. The average values for the Class 0 and Class I sources in red and blue, respectively. The average ratios are taken in 1 km s^-1 bins.