Number of sources retrieved in the I band vs. [I] in bins of 0.5 mag. The boxes in each row refer to the Spitzer/IRAC band I indicated on the upper left corner of the left-hand box. Upper two rows: in the left column, we compare the I-band sources detected in all IRAC bands with photometric errors <0.3 mag (black line), and the sources detected in at least the first two IRAC bands with photometric errors <0.3 mag (red line); in the right column, we compare all sources detected in at least the first two IRAC bands, with photometric errors either <0.3 mag in both bands (black line), or <0.3 mag in at least the considered band I (blue line). Lower two rows: in the left column, we compare the I-band sources detected in all IRAC bands, with photometric errors either <0.3 mag in all bands (black line), or <0.3 mag in at least the considered band I (red line); in the right column, we compare the I-band sources detected in all IRAC bands, with photometric errors <0.3 mag in at least the considered band I (black line), and the sources detected in at least the considered band I with photometric errors <0.3 mag (blue line).