reduction of $\chi^2$ similar to the reduction for HERAPDF2.0 would have been obtained. However, the instability of the (15+4)-parameter fits was considered to outweigh this minimal gain in $\chi^2$. The $\chi^2/{\rm dof}$ values of all ZEUS-EW fits are similar to the values for ZEUS-13p. As 14-parameter fits were used to evaluate the parameterisation uncertainties, the uncertainties associated with the $D_{\bar{U}}$ are included.

All results were cross-checked with fits at NNLO QCD, which yielded compatible results. However, as the EW analysis is partially at LO, see below, a treatment of the PDFs at NLO was considered more consistent, because $\alpha_s^2$ is of the same order of magnitude as $\alpha$.

The uncertainties on the polarisation as listed in Table \[tab:pol\] were taken into account in all fits presented in this paper. However, it was found that the effect of these uncertainties is negligible compared to the total experimental/fit uncertainty.

As described in Section \[sec-pred\], the reduced cross sections used as input to the analysis were published by the individual collaborations after QED corrections were applied. These corrections are mostly on the percent level, but reach 15% for a few cross sections. The correction factors were calculated by producing Monte Carlo data sets 