Deviation of rms as a function of the average magnitude for all the selected sources in the CDF-S3 (left panel) and 4 (right panel) detected in at least 20% of epochs. Cyan crosses: selected sources. Solid red line: variability threshold (95th percentile of the rms distribution). Dashed red line: running average of the rms deviation. Black points: variables sources (i.e., above the defined variability threshold).