Top: difference between pPXF measured velocity dispersions (σ) and input velocity dispersions in simulated spectra (σ_in), as a function of σ. Only the galaxies with ⟨S/N⟩ > 60 are shown here. The data points are color shaded according to the spectra ⟨S/N⟩. Black triangles are the medians of the distributions in 7 different σ bins. The black polynomial line underlines the small increasing bias at low σ. Bottom: black solid (blue dashed) line shows the relative statistical error estimated from simulations (provided by pPXF), as a function of ⟨S/N⟩. The red curve corresponds to the ratio between the two; the statistical relative errors appear to be underestimated by ∼20% in our spectra.