On the Impact of Cepheid Outliers on the Distance Ladder
  Recent work by Efstathiou (2014) highlighted the importance of outliers in
the period-luminosity (PL) relation of Cepheid data on the distance ladder. We
present a statistical framework designed to address this difficulty, and apply
it to the Cepheid data from the Milky Way (MW), the Large Magellanic Cloud
(LMC), and the Riess et al. (2011) (hereafter R11) dataset. We consider two
possible models of the outlier population in the R11 Cepheid dataset. One of
these models exhibits tension between the PL relation of the R11 cepheids and
the MW+LMC cepheids, while the other does not. We extend our models to
adequately account for tension between the cepheid data sets when appropriate.
Our outlier treatment has a significant impact on the distance scales to
Supernovae hosts with Cepheid distances, increasing the uncertainty in these
distances by a median factor of ~30%. We further find that our Cepheid outlier
treatment translates into a modest, but non-negligible increase in the
statistical uncertainty of H0, adding in quadrature 1.2 km/s/Mpc. Combined with
the increased scatter in the Hubble diagram reported by Jones et al. (2015), we
find H0=72.6+/-2.8 km/s/Mpc, corresponding to a 3.8% uncertainty in H0 from
local measurements. This value is fully consistent with both the Planck and
inverse-distance ladder H0 constraints.
