  Using published HST Cepheid data from 25 galaxies, we have found a
correlation between the P-L dispersion and host galaxy metallicity which is
significant at the ~3sigma level in the V band. This result supports the
suggestion that the metallicity dependence of the Cepheid P-L relation may be
stronger than expected. The high observed dispersions in the HST Cepheid P-L
relations have the further consequence that the bias due to incompleteness in
the P-L relation at faint magnitudes is more significant than previously
thought. In the HST sample, galaxies at higher distance tend to have higher
metallicity. This means that when a full metallicity/incompleteness correction
is made, a scale error in the published Cepheid distances is seen in the sense
that the published distance moduli are increasingly underestimates at larger
distances. This results in the average distance modulus to the four galaxies in
the Virgo cluster core increasing from (m-M)_0=31.2+-0.19 to (m-M)_0=31.8+-0.17
with similar increases for the Fornax and Ursa Major clusters. For the 18 HST
galaxies with good Tully-Fisher distances and (m-M)_0>29.5 the Cepheid-TF
distance modulus average residual increases from 0.44+-0.09 mag to 0.82+-0.1mag
indicating a significant scale error in TF distances and resulting in the
previous Pierce & Tully TF estimate of H_0=85 reducing to H_0=58+-7. Finally,
for the 8 HST galaxies with SNIa, the metallicity corrected Cepheid distances
now imply a metallicity dependence of SNIa peak luminosity in the sense that
metal-poor hosts have lower luminosity SNIa. Thus SNIa Hubble diagram estimates
of both H_0 and q_0 may also require significant metallicity corrections.
