Uncertainty in the extreme ultraviolet spectrum of a 1 Myr starburst as predicted by a range of stellar population synthesis codes, given the same two metallicities and (α_u = α_m = −2.35) IMF, but different stellar evolution and atmosphere input models. High resolution models have been slightly smoothed for display purposes. Model references: BPASS (Eldridge et al. 2017), BC03 (Bruzual & Charlot 2003), Starburst99 (Leitherer et al. 1999, 2011), Yggdrasil (Zackrisson et al. 2011). The 2016 version of the BC03 models has not been fully described in the refereed literature but some details are to be found in Chevallard & Charlot (2016) and Wofford et al. (2016).