Type Ia supernovae selection and forecast of cosmology constraints for the Dark Energy Survey

► We study the purity of a Type Ia supernova sample, as a function of selection cuts. ► We present and expound on improved simulations of core collapse background samples. ► We evaluate the Dark Energy Task Force Figure of Merit for all analysis variations. ► We show notable factors altering the Figure of Merit (FOM), with cosmology analyses. ► The FOM is more affected by reduced purity than by the number of Type Ia supernovae.