  Triggered by ongoing experimental analyses, we report on a study of the cross
section ratio sigma(pp -> ttbb)/sigma(pp -> ttjj) at the next-to-leading order
in QCD, focusing on both present and future collider energies: sqrt{s}= 7, 8,
13 TeV. In particular, we provide a comparison between our predictions and the
currently available CMS data for the 8 TeV run. We further analyse the
kinematics and scale uncertainties of the two processes for a single set of
parton distribution functions, with the goal of assessing possible correlations
that might help to reduce the theoretical error of the ratio and thus enhance
the predictive power of this observable. We argue that the different jet
kinematics makes the ttbb and ttjj processes uncorrelated in several
observables, and show that the scale uncertainty is not significantly reduced
when taking the ratio of the cross sections.
