Distribution of the logarithm of the absolute trend β_P (leftmost panel), the frequency of the oscillatory term f_P (second panel) and the logarithm of the uncorrected and corrected relative amplitude of the modulation, that is, the amplitudes ΔP and ΔP_c of the oscillatory term divided by the catalog period P_cat (third and rightmost panels) in model (2), for the changes in the pulsation period of all Cepheids in our sample. Thecolors in the twin-peak groups indicate the absolute value of the separation of the twin peaks, ranging from light blue for small separations to dark blue for large ones. The label T.FU and C.FU corresponds to fundamental-mode twin-peak and control Cepheids, respectively; similarly, the labels T.FO and C.FO indicate first overtone twin-peak and control cepheids, respectively. The T.FO group includes the 5 cepheids selected because of their strong amplitude changes.