a) Dependence of ΔICF(He)= ICF(EW(Hβ)) – ICF(SED) on the oxygen abundance 12 + log O/H for 64 SDSS galaxies with EW(Hβ) ≥ 100 Å, with the excitation parameter x = O^2 +/O ≥ 0.7 and with 1σ error in Y ≤ 3% (dots) and for 32 SDSS galaxies with EW(Hβ) ≥ 150 Å, with the excitation parameter x = O^2 +/O ≥ 0.8 and with 1σ error in Y ≤ 3% (encircled dots). Here ICF(EW(Hβ)) is the ionisation correction factor derived for a starburst age estimated from EW(Hβ), while ICF(SED) is the ionisation correction factor for the starburst age, which is derived from the spectral energy distribution (SED) fitting. Horizontal solid and dashed lines are average values of ΔICF(He) for dots and encircled dots, respectively. b) The dependence of ΔICF(He) = ICF(EW(Hβ)) – ICF(SED) on the Hβ equivalent width EW(Hβ). Symbols and lines have the same meaning as in a). c) The dependence of ΔICF(He) = ICF(EW(Hβ)) – ICF(SED) on the excitation parameter x. Symbols and lines have the same meaning as in a).