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<font face="Arial, Helvetica"> Spectroscopic and photometric properties of low and high-z supernovae Ia (SNe Ia) have been analyzed in order to achieve a better understanding of their diversity and to identify possible SN Ia sub-types. <br /> We use a wavelet based approach to obtain wavelet transformed spectra on which one can easily measure <br /> spectral features. We exemplify it with the Si II 4000 equivalent width. The ability and, especially, the <br /> facility to extend the method to SNe at high-z is demonstrated. We applied the method to 110 SNe Ia and found correlations between $EW_w \lbrace \rm{Si II}\rbrace$ and parameters related <br /> with the light-curve shape for 88 supernovae with available photometry. No evidence for evolution of $EW_w \lbrace \rm{Si II}\rbrace$ with redshift is seen. The three sub-classes of SNe Ia were <br /> con?rmed using an independent cluster analysis with only light-curve shape, colour, and $EW_w \lbrace \rm{Si II}\rbrace$. SNe from high-$z$ sample seem to follow a similar grouping as nearby <br /> objects. The $EW_w \lbrace \rm{Si II}\rbrace$ value measured on a single spectrum may point towards SN Ia sub-classi?cation, avoiding the need for the expansion velocity gradient calculations as is commonly used. Further implications for SNe Ia cosmology are discussed. </font>