Abundance patterns in stars can be used to build up information about the stellar populations in star clusters and galaxies. This can lead to determinations of when the stars were made, their chemical abundances and the duration of star formation. All these characteristics are important for understanding galaxy evolution. So far such determinations have largely been achieved using base stellar libraries, plus theoretical models of how star spectra change with element abundances in the models (i.e. theoretical response functions). Our work uses measurements of [Mg/Fe], in an observed stellar library, to test these theoretical response functions. This talk presents the results and discusses some of the major issues in stellar population analysis.