Bias analysis of the sample. Left panel: Relation between the luminosity at 3000Å and redshift. The grey patch indicates the redshift bin we used to test the effect of the S/N (see Appendix A). The colour code marks the S/N value in the log scale. Middle panel: Behavior of the S/N in each luminosity bin. The colour scale describes the S/N, and the number in parentheses corresponds to the number of sources in each bin. Right panel: Distribution of the S/N in each luminosity bin. The numbers in parentheses indicate the results from a two-sample KS test.