Fig. 1. The difference between the empirical Cepheid distance by KP studies and the predicted value with , plotted against the O/H metallicity of the host galaxy. The Cepheid samples are original KP data (open circles) or refer to SNP galaxies extracted from the HST archive and re-processed by the KP team (filled circles). The two dashed lines depict a discrepancy on the distance of the order of 10%.