Relative contribution of galaxy stochasticity to the EoR history. In the left panel we show the common approximation of an EoR history calculated assuming a constant clumping factor (cf. Eq. (11)), while in the right we show the fraction of 5 cMpc regions that exceed the threshold of two ionizing photons per baryon per age of the Universe (cf. Fig. 5). Black curves correspond to our fiducial model, including all sources of scatter. Green/blue curves ignore scatter around the SHMR/SFMS, while the orange curves ignore all sources of scatter. We see that not accounting for the burstiness of star formation (that is, scatter around the SFMS) can result in EoR histories that are delayed by Δz ∼ 0.5–1. Not accounting for any scatter and assuming only mean galaxy properties delays the completion of the EoR by Δz ∼ 2.