Normalised Lomb-Scargle periodogram (LSP), structure function (SF) and multiple fragments variance function (MFVF) (from top to bottom). The green lines/points are the measured results of the light curve of PKS 2155-304. The histograms in colour scale represent the PDFs obtained from simulated light curves with the best-fit parameter sets (β = 5.0,   f_min = 10^-3.2 d^-1), (β = 1.6,   f_min = 10^-3.4 d^-1) and (β = 1.8,   f_min = 10^-3.0 d^-1) respectively. Only the bins at f < 0.05   d^-1 are used for the maximum-likelihood method.