Extraction of the proton parton density functions using a NLO-QCD fit of
the combined H1 and ZEUS inclusive DIS cross sections

The combined HERA-I data set, of neutral and charged current 
inclusive cross-sections for $e^+p$ and $e^-p$ scattering, is used as the
sole 
input for an NLO QCD PDF fit in the DGLAP formalism. 
The consistent treatment of systematic uncertainties in the joint data set 
ensures that experimental uncertainties on the PDFs can be calculated
without 
need for an increased $\chi^2$ tolerance. This results in PDFs with greatly

reduced experimental uncertainties compared to the separate analyses of 
the ZEUS and H1 experiments. Model uncertainties, including those 
arising from parametrization dependence, are also carefully considered. The

resulting HERA-I PDFs have impressive precision compared to the global
fits.