Fraction of simulated objects with successful spectroscopic redshift as a function of m_VIS. The lines represent the completeness fraction of the spectroscopic training samples. The blue line corresponds to the fraction of objects for a random training subsample that is fully representative of the sample under study. In black we show an expectation of the spectroscopic completeness for future ground-based surveys in m_VIS (see text). In orange we present the completeness of a training sample with an n(z) similar to the currently available spectroscopic data (see text). The number of objects included in each training set is not represented by the normalization of the different curves in this figure (see Fig. 2 for the redshift distributions). Although our photometric samples go up to m_VIS = 25, we cut the spectroscopic training samples at m_VIS < 24.5 because realistic redshifts have not been reliably determined beyond that magnitude limit yet.