  We use the spectra of ~ 22,000 early-type galaxies, selected from the Sloan
Digital Sky Survey, to infer the ages, metallicities and star formation
histories of these galaxies. We find clear evidence of "downsizing", i.e.
galaxies with larger velocity dispersion have older stellar populations. In
particular, most early-type galaxies with velocity dispersion exceeding 200 km
s-1 formed more than 90% of their current stellar mass at redshift z > 2.5.
Therefore, star formation was suppressed around this redshift. We also show
that chemical enrichment was rapid, lasting 1-2 Gyr and find evidence that
[Fe/H] is sub-solar. We study the robustness of these results by comparing
three different approaches: using (i) Lick absorption line indices; (ii)
fitting a single-burst stellar population model to the whole spectrum
(lines+continuum); and (iii) reconstructing the star formation and metallicity
histories in multiple age-bins, providing a method to measure mass-weighted
ages and metallicities. We find good agreement between the luminosity-weighted
ages and metallicities computed with these three methods.
