Top panel: distribution of 1.4–3 GHz spectral indices for sources detected at both frequencies (green line), and only at 3 GHz (lower limits, blue line) or 1.4 GHz (upper limits, red line). Middle panel: cumulative distribution function (CDF; black line) and error estimate (gray shaded area) of spectral indices calculated using the survival analysis also taking lower limits into account. The red dashed line shows a linear extrapolation of the distribution to zero assuming the maximum theoretically attainable spectral index of α = 2.5 (see text for details). Bottom panel: probability density function (PDF) for spectral indices calculated as a first derivative of the CDF extrapolated to 0 at high end (middle panel). A Gaussian fit to the distribution is also shown (red curve) and its mean and standard deviation are indicated in the panel.