SSA applied to PG 1553+113. Blue dots represent the original signal with error bars. Orange, green, and red lines correspond to the periodicity component, the trend, and the sum of the trend and the periodicity reported by the SSA algorithm, respectively. The spacing between the blue bars indicates the period computed through the LSP (2.2 yr). The width of the blue bars is the period uncertainty (0.2 yr) computed from the FWHM of the period peak in the LSP.