Distributions of simulated SNe Ia (normal) interacting with a CSM with respect to the flux ratio f_V. The objects classified correctly have larger f_V ratios and the two distributions are statistically different at a high significance (p ~ 10^-15 in a KS test). All spectra that were given a Ia-CSM designation have been considered to be correctly classified. The dashed line indicates that less than 16% of the simulated events were correctly (wrongly) classified at flux ratios below (above) f_V = 0.3.