It is important to ensure that all detectors behave as expected and that related observables (a couple of examples: Number of Tracks in TPC, dE/dx of electrons, Number of calorimeter clusters found,...) are described properly by MC simulations. In particular, it is important to check that any of these observables is the same for all the subsets of the data, namely for all different runs that were recorded. If this is not the case, the differences must be understood in case the MC does not follow.