The upper panel shows the period distribution for the Cepheids in the full Berdnikov catalogue (455 Cepheids) as well as the volume-complete sample within the assumed completeness limit of rcosb < 1 kpc (71 Cepheids). Note the larger fraction of long-period Cepheids in the full Berdnikov catalogue due to selection effects. The line histogram in lower panel shows our modelled period distribution consisting of two overlapping Gaussians, which ideally should represent the volume-complete sample in the upper panel. In the model distribution, 96% of the population has a normal distribution of log P with mean value 0.75 and standard deviation 0.18, and 4% has a normal distribution with mean value 1.48 and standard deviation 0.20.