  This note presents Monte Carlo generator comparisons of the ttbb and ttW
processes at particle level. The aim is to compare the modelling of important
backgrounds to ttH measurements in multi-lepton final states and in the
ttH(H->bb) decay channel and the treatment of the associated theory
uncertainties for a combination of the full Run-2 ttH results from ATLAS and
CMS. As a first step, modelling and theory uncertainties as used in ATLAS an
CMS are compared in the relevant analysis regions. Significant differences in
the treatment of systematic uncertainties between the experiments have been
observed in ttbb and ttW. As a first step, ATLAS and CMS agreed on a common
reference value of the inclusive ttW cross section to allow direct comparisons
between experiments.
