Number of required populations, averaged over 100 Monte-Carlo realisations, as a function of metallicity. For each realisation, the significance of the χ²-distribution is evaluated, and the likelihood of requiring one or two populations is evaluated. Blue (black) line represents the results for the low-α (high-α) population. A value larger than 1.5 indicates that the low-α (high-α) population alone cannot describe the observed trends.