Comparison of the HαEW age indicator of BPASS single-star (Eldridge & Stanway 2009), GALEV (Kotulla et al. 2009), and Starburst99 (Leitherer et al. 1999) SSPs, illustrating the differences between various SSP models. BPASS and GALEV HαEWs were measured directly from the SSP model spectra (see Sect. 3.1 for the measurement method), while in Starburst99 the values are given in tabulated form. All SSP models assume instantaneous star formation and a standard Salpeter IMF. As HαEW also depends on the metallicity of the SSP model, two different metallicities are plotted to illustrate this effect. The solid lines indicate solar metallicity (Z = 0.02) models and the dashed lines models with Z = 0.004.