Pulsation period distribution for the parent OGLE sample (dashed lines) and irregular period change candidates (class 3, solid lines). From top to bottom, the panels show the distributions for LMC F, SMC F, LMC 1O, and SMC 1O mode Cepheids, respectively. The incidence rate for irregular period change candidates is displayed for bins containing at least 10 Cepheids and with an incidence rate exceeding 5%.