Comparison and combination of ZEUS and H1 PDF analyses
  The H1 and ZEUS published PDF analyses are compared, including a discussion of the different treatments of correlated systematic uncertainties. Differences in the data sets and the analyses are investigated by putting the H1 data set through both PDF analyses and by putting the ZEUS and H1 data sets through the same (ZEUS) analysis, separately. Finally the HERA averaged data set is put through the ZEUS PDF analysis and the result is compared to that obtained when putting the ZEUS and H1 data sets through this analysis together, using both the   Offset and the Hessian methods of treating correlated systematic uncertainties. 
A. Cooper-Sarkar, C. Gwenlan