Relative numbers of massive stars ℛ formed within multiple initial mass ranges as a function of power-law exponent α₃ for the high-mass IMF component (M > 1 M_⊙). See Eq. (6) for the exact definition of ℛ. Upper panel: same SFR, irrespective of the IMF changes. The number of stars is thus calculated with respect to a fixed stellar mass formed. Lower panel: SFR corrected for the IMF changes (see Eq. (5)). The number of stars is thus calculated with respect to a fixed amount of UV radiation at 1500 Å wavelength that is constrained by the observations (Madau & Dickinson 2014).