  Using the third data release of the Sloan Digital Sky Survey (SDSS) we have
rigorously defined a volume limited sample of early-type galaxies in the
redshift range z < 0.1. We have defined the density of the local environment
for each galaxy using a method which takes account of the redshift bias
introduced by survey boundaries if traditional methods are used. At
luminosities greater than our absolute r-band magnitude cutoff of -20.45 the
mean density of environment shows no trend with redshift. We calculate the Lick
indices for the entire sample and correct for aperture effects and velocity
dispersion in a model independent way. Although we find no dependence of
redshift or luminosity with environment we do find that the mean velocity
dispersion, sigma, of early-type galaxies in dense environments tends to be
higher than in low density environments. Taking account of this effect we find
that several indices show small but very significant trends with environment
that are not the result of the correlation between indices and velocity
dispersion. The statistical significance of the data is sufficiently high to
reveal that models accounting only for alpha-enhancement struggle to produce a
consistent picture of age and metallicity of the sample galaxies, whereas a
model that also includes carbon enhancement fares much better. We find that
early-type galaxies in the field are younger than those in environments typical
of clusters but that neither metallicity, alpha-enhancement nor carbon
enhancement are influenced by the environment. The youngest early-type galaxies
in both field and cluster environments are those with the lowest sigma.
However, there is some evidence that the objects with the largest sigma are
slightly younger, especially in denser environments.
