Distributions of the peak to 3σ ratio and its relation with outflow detection rate for clumps in different stages. Top panel: the cumulative distributions of the ratio between the peak of ¹³CO and 3σ noise level of each corresponding spectrum (peak ¹³CO (2–1) T_mb∕3σ) for clumps in the four evolutionary stages in the total 2052 sample. Bottom panel: outflow detection rate as a function of the (peak ¹³CO (2–1) T_mb∕3σ). This indicates that outflow wings are stronger for clumps in the late evolutionary stages and thus easier to detect, compared to clumps in the early evolutionary stages. The detection rates of outflows are likely to be similar for clumps in every evolutionary stage. The error bars on the x-axis and y-axis are determined following the same method in Fig. 6.