Distribution of SFs divided by $\sqrt{2}$ and the original photometric uncertainty (σ_o) of the g-band light curves of the non-variable sources. The solid blue line denote the median value (0.90) and the dashed lines give its median absolute deviation (0.06). If the photometric noise is estimated robustly, the median value should be equal to 1; the median value of 0.90 indicates that the photometric uncertainty is underestimated.