Y – O/H for the sample of 111 H ii regions with EW(Hβ) ≥ 150 Å, with the excitation parameter x = O²⁺/O ≥ 0.8 and with 1σ error in Y ≤ 3%. The five He i emission lines λ3889, λ4471, λ5876, λ6678, and λ7065 are used for χ² minimisation and determination of Y. Large blue and green filled circles are for HeBCD and VLT samples, respectively, 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) and the dashed lines are 1σ alternatives of the linear regression. The dotted line is the quadratic maximum-likelihood fit to the data.