Crossover error rate (CER) when attempting to distinguish α-rich and α-poor spectra when imposing a threshold [α/Fe] on the estimates derived during cross-validation of the synthetic sample. The black region around 6000 K has a CER near 50%, indicating that the α-rich and α-poor spectra are practically indistinguishable. The pale yellow region around 4000 K has a CER below 10%, indicating that the α-rich and α-poor spectra can easily be told apart.