Relative change of the signal efficiency ϵsig during the iterative removal of the least important variable (from bottom to top). The signal efficiency for each training is compared to the signal efficiency of the reference training ${ϵ}_{\text{sig}}^{\text{HillasOnly }}$ (the current training with the 6 Hillas variables) for a background rejection of 95%. The black error bars show the difference in the signal efficiency for simulations with nominal and high NSB values, which exceeds the difference present in the reference training. No error bar corresponds to a difference which is smaller than in the reference training. Variables below the dashed red line are not selected for the final training.