Fitting the PL relation of the fundamental-mode Cepheids in M 51 above the period limit of P > 10 days. The open yellow circles show the NGC 4258 Cepheids for reference, and the faint grey points denote the stars that were rejected by the outlier-detection method. The red line corresponds to the MAP estimate, i.e. the most likely fit. The displayed distance uncertainty includes the systematic terms (namely the uncertainty in the metallicity correction and the reference NGC 4258 PL relation).