a)Y – O/H for the sample of 1610 H ii regions. The five He i emission lines λ3889, λ4471, λ5876, λ6678, and λ7065 are used for the χ² minimisation and the determination of Y. Large blue and green filled circles are for the HeBCD and VLT samples, small red filled circles are SDSS galaxies. We chose to let T_e(He⁺) vary freely in the range 0.95−1.05 of the (He⁺) value. The continuous line represents the linear regression (whose equation is given at the bottom of the panel). b) Same as a), but only H ii regions with EW(Hβ) ≥ 100 Å are shown, with the excitation parameter x = O²⁺/O ≥ 0.7 and with 1σ error in Y ≤ 3%. The dotted line is the quadratic maximum-likelihood fit to the data. c) Distribution of the best derived values T_e(He⁺)/T_e(O iii) versus T_e(O iii) for the case shown in b). d) Distribution of the best derived values N_e(He⁺) versus N_e(S ii) for the case shown in b).