Trend of the standard deviation of the residuals between the flux of the ISM residual spectrum beyond the DIB region and that of the continuum as a function of S/N in the upper panels, and T_eff in the lower panels. Three distinct colors encode the three DIBs λ5780, λ5797, and λ6614. In each panel, a violin plot is used to present the distribution of the standard deviation of the residuals for the binned HQ samples. The median of each bin is annotated with a white dot, and the 2.5 and 97.5 percentiles are marked with black dots. The red dashed line connects the median of each bin to show the trend of the standard deviation of the residuals.