2D distribution of the spectral index for all quasars in the study. The four quadrants represent different shapes of the radio spectra. The blue stars in the lower-right quadrant are the MPS samples discussed in this paper. Red dots indicate sources that meet $0<{\alpha }_{54}^{144}<0.1$ and ${\alpha }_{144}^{1400}<0$ criteria, and light blue dots represent the remaining samples. The horizontal solid, vertical solid, vertical dashed, and horizontal dashed gray lines represent the ${\alpha }_{144}^{1400}=0$, ${\alpha }_{54}^{144}=0.1$, ${\alpha }_{54}^{144}=0,$ and 1-to-1 lines. The typical median error of samples is represented by black lines in the bottom-right corner. The median errors are 0.09 and 0.03 for ${\alpha }_{54}^{144}$ and ${\alpha }_{144}^{1400}$, respectively. Top and right: Histogram of the spectral index, with colors corresponding to those in the middle panel. The vertical lines represent the median values of each sample.