The χ² diagnostic for the I-band LF fit for 2 burst simulations, built using alpha-enhanced stellar evolutionary models, as compared to observations is plotted as a function of the old (x-axis) + young (y-axis) population age. Each panel shows different relative fractions of old + young population in the combined 2-burst simulation. The size and colour of the points are normalized to the full range of the χ² values of the LF_I fit for all 2-burst models with input observed MDF. The larger the symbol, and darker blue its colour, the smaller the χ².