2014MNRAS.440.1138E

I reanalyse the Riess et al. (hereafter) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (hereafter). I explore different outlier rejection criteria designed to give a reduced χ2 of unity and compare the results with the rejection algorithm, which produces a reduced χ2 that is substantially less than unity and, in some cases, leads to underestimates of the errors on parameters. I show that there are sub-luminous low-metallicity Cepheids in the sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud as an anchor. I also show that the small Milky Way Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6±3.3 km/s/Mpc, compatible within 1σ with the recent determination from Planck for the base six-parameter Λ cold dark matter cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5±2.5 km/s/Mpc, which differs by 1.9σ from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage.