3 GHz to 1.4 GHz spectral indices () of the star-forming population as a function of redshift. Red points show direct measurements, while yellow triangles show 5σ lower limits for objects not detected at 1.4 GHz. The median within redshift bins are shown by black squares and have been calculated using a single-censored survival analysis, which incorporates the lower limits. The median of the star-forming population is consistent with (indicated by the horizontal dashed line), at least at z ≲ 2.