Distribution with the oxygen abundance of the empirically derived weighted mean Ya) and N_eb) values calculated with the Porter et al. (2013) He i emissivities. Nine He i λ3889, λ4026, λ4388, λ4471, λ4922, λ5876, λ6678, λ7065, and λ10830 emission lines are used for χ² minimisation and determination of Y. In the lower parts of the panels showing Y as a function of O/H we indicate the mean of all Y values derived from the models, together with the dispersion. Top: the electron temperature T_e(He⁺) is randomly varied in the range (He⁺) where (He⁺) is defined by Eq. (32), and the best derived values of T_e(He⁺) for every model are shown in c). The solid line in a) shows the input CLOUDY Y value of 0.254, the dotted lines are 1% deviations, and ⟨ Y ⟩ is the average value of Ys shown by filled circles. Middle: the same as in the top panel, but T_e(He⁺) = (He⁺). Bottom: the same as in the top panel, but T_e(He⁺) = T_e(O iii).