Two (13+1)-parameter fits called ZEUS-EW-S and ZEUS-EW-W were used to extract $\sin^2\theta_W$ and $M_W$ separately, while keeping the other one fixed. In addition, a (13+2)-parameter fit called ZEUS-EW-S-W was performed to extract simultaneously $\sin^2\theta_W$ and $M_W$. As cross-checks, fits in which the PDF parameters were fixed to ZEUS-13p and only the electroweak parameters were allowed to vary were also performed.

The parameterisation uncertainties for all fits were obtained by adding extra $D$ and $E$ parameters one by one to the fit. It was checked whether this caused a significant change in the result on the EW parameters. It turned out that only adding back the parameter $D_{\bar{U}}$ or adding the parameter $D_g$ resulted in significant differences. If a (14+4)-parameter fit including the parameter $D_{\bar{U}}$ would have been chosen for ZEUS-EW, the determination of the parameterisation uncertainties would have required (15+4)-parameter fits. Such fits were found to be too unstable to provide reliable uncertainties.

The parameter $D_{\bar{U}}$ was added for the extraction of HERAPDF2.0 because it reduced the overall $\chi^2$ by about 0.005 per degree of freedom to 1357/1131=1.200. The $\chi^2$ per degree of freedom of ZEUS-13p is $3275/2929 =1.118$. If $D_{\bar{U}}$ would have been added as a 14th parameter, a 