Behavior of line-strength indices. From top to bottom, the distribution of FASTAR semi-resolved predictions as a function of the number of stars in the population is shown for the H_βO, Mgb, and TiO₂ features, respectively. These distributions correspond to 1,000,000 stochastic IMF samplings of the same 10 Gyr old, solar metallicity, and Milky Way-like IMF population. The median of the distribution is shown with a solid blue line and the shaded regions correspond to the 10/90 and 1/99 percentiles. The blue dashed line indicates the fully sampled SSP FASTAR prediction. For comparison, dashed horizontal lines mark the line-strength value expected in the fully sampled regime for various ages, metallicities and IMFs. Line-strength values have been calculated at the nominal 2.51 Å resolution of the FASTAR models.