Sensitivity to changes in the stellar population properties. Left: Weights distribution retrieved after fitting the MILES SSP model prediction of a population with solar metallicity and an age of 5 Gyr, with two isochrones of 5 and 10 Gyr overplotted for comparison. Middle: Similar weights distribution but for an old (10 Gyr) solar metallicity population with a relative excess of low-mass stars (i.e., a bottom-heavy IMF). In this case, the isochrone corresponds to a 10 Gyr population. Right: Recovered metallicity weights after fitting two MILES SSP models, in red and blue for a metal-rich and a metal-poor population, respectively.