Selection of sources for the cross-match for the GMRT 610 MHz (left panel) and the NVSS (right panel) samples. All the cross-matched sources are shown as blue dots and the finally selected ones are marked with an orange cross. The completeness limit of the survey is shown as a vertical dashed line. The line at which the flux densities are equal is shown as reference as a solid green line. The locus of sources with a spectral index of 1.5 is shown as a dash-dotted red line. The point at which this line and the survey completeness line cross is used as a reference for the selection threshold in the product of flux densities (see text) which is marked as a purple dotted line.