  (Abridged) Environmental differences in the stellar populations of early-type
galaxies are explored using principal component analysis (PCA), focusing on
differences between elliptical galaxies in Hickson Compact Groups (HCGs) and in
the field. The method is model-independent and relies on variations between the
observed spectra. The projections (PC1,PC2) reveal a difference with respect to
environment, with a wider range in PC1 and PC2 in the group sample. We define a
spectral parameter (zeta=0.36PC1-PC2) which simplifies this result to a single
number: field galaxies have a very similar value of zeta, whereas HCG galaxies
span a wide range in this parameter. We obtain a strong correlation between the
values of zeta and the mass fraction in younger stars, so that some group
galaxies present a higher fraction of them. PCA is more sensitive than other
methods based on a direct analysis of observables such as the structure of the
surface brightness profile or the equivalent width of absorption lines. The
latter do not reveal any significant variation between field and compact group
galaxies. Our results imply that the presence of young stars only amounts to a
fraction of a percent in its contribution to the total variance.
