σ_I vs. I diagram for the 4646 stars studied in this work. The solid curve is used to separate real from spurious variables (as proposed by Graczyk & Eyer 2010), while the region between the solid and the dashed curves defines the low-amplitude variables. A zoomed-in region is presented in the lower panel. The irregular variables (crosses), the Cepheids (inverted triangles), and most of the EBs (open circles) lie above the dashed curve.