Distribution of variability σ_F/μ_F (standard deviation of F divided by its mean) for the 1430 objects with more than one observation. The histogram shows four groups of different timespan ranges ▵T in days. Objects with the longest timespan (white) show small variability, while some of those with the shortest timespan (black) can exhibit significant changes. Objects within the group of the third longest timespan (light grey) display the strongest variation of their Hα profile.