  We develop a method to measure the strength of the absorption features in
Type Ia supernova (SN Ia) spectra and use it to make a quantitative comparison
between the spectra of Type Ia supernovae at low and high redshifts. In this
case study, we apply the method to 12 high-redshift (0.212 < z < 0.912) SNe Ia
observed by the Supernova Cosmology Project . Through measurements of the
strengths of these features and of the blueshift of the absorption minimum in
Ca II H&K, we show that the spectra of the high-redshift SNe Ia are
quantitatively similar to spectra of nearby SNe Ia (z < 0.15). One supernova in
our high redshift sample, SN 2002fd at z=0.279, is found to have spectral
characteristics that are associated with peculiar SN 1991T/SN 1999aa-like
supernovae.
