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We present FEROS-Lick/SDSS, an empirical data base of Lick/SDSS spectral indices of FGK stars to be used in population synthesis projects for discriminating different stellar populations within the integrated light of galaxies and globular clusters. From about 2500 FEROS stellar spectra obtained from the European Southern Observatory Science Archive Facility, we computed line-strength indices for 1085 non-supergiant stars with atmospheric parameter estimates from the AMBRE project. Two samples of 312 dwarfs and of 83 subgiants with solar chemical composition and no significant α-element abundance enhancement are used to compare their observational indices with the predictions of the Lick/SDSS library of synthetic indices. In general, the synthetic library reproduces very well the behaviour of observational indices as a function of temperature, but in the case of low-temperature (Teff ≲ 5000 K) dwarfs; low-temperature subgiants are not numerous enough to derive any conclusion. Several possible causes of the disagreement are discussed and promising theoretical improvements are presented. [ABSTRACT FROM AUTHOR]