Difference between the intra-band spectral index (α_LOFAR) and the NVSS to LOFAR spectral index (α_LOFAR-NVSS) compared to the S/N of the cross-matched sources. The distribution of S/N is divided in deciles and the median of the difference is computed and shown as orange lines. The spectral indices are flatter (lower values of the difference) for low S/N likely due to incompletness effects. The distribution is close to zero (marked as a horizontal grey line) for sources with S∕N > 500 (shown as a vertical green dashed line).