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tokens: there is a normally distributed scatter around the mean gas metallicity given by the MZR, with the dispersion σ 0 = 0.1 dex (the intrinsic scatter in the MZR). We allow σ 0 to increase linearly with decreasing mass at M * 10 9.5 M ⊙ . 2.3.3 Distribution of metallicity within galaxies The average metallicity of a galaxy roughly corresponds to the metal content that would be measured in this galaxy at the distance of one effective radius from its centre (Kewley Ellison 2008 ). Due to metallicity gradients present within galaxies there is a certain range of metallicities with which stars can form inside their host. The azimuthal metallicity variations are typically negligible (e.g. Sánchez-Menguiano et al. 2017 ). However, typical radial abundance gradients in the local disc galaxies are around −0.1 dex/ r e (∼−0.03 dex kpc− 1 ) (e.g. Sánchez et al. 2014 ; Sánchez-Menguiano et al. 2016 ). The detailed studies of the distribution of metallicities of H ii regions within z ∼ 0 disc galaxies carried out by Sánchez et al. ( 2014 ) and Sánchez-Menguiano et al. ( 2016 ) with the CALIFA survey show that the distribution of metallicities at which the star formation proceeds is roughly symmetric with respect to the average value found at . However, the exact range of metallicities of the H ii regions and the metallicity gradient depend on the assumed metallicity calibration (see e.g. table 1 of Sánchez-Menguiano et al. 2016 ). 2 Our choice : We assume that the distribution of metallicities at which the star formation proceeds within galaxies can be represented by a normal distribution with dispersion = 0.14 dex. This particular choice of corresponds to half of the average range of metallicities of H ii regions 2 found by Sánchez et al. ( 2014 ) and Sánchez-Menguiano et al. ( 2016 ), averaged between the two studies. See Appendix E for additional discussion. 2.4 Star formation–mass relation The stellar mass and star formation rate of star-forming galaxies are strongly correlated, giving rise to the star formation–mass relation (SFMR; e.g. Brinchmann et al. 2004 ; Salim et al. 2007 ; Speagle et al. 2014 ; Tomczak et al. 2016 ; Boogaard et al. 2018 ) also called the star-forming main sequence. SFRs are measured based on the luminosities of galaxies observed in certain bands that correlate with the recent (≲100 Myr) star formation activity. Those luminosities need to be corrected for dust attenuation, which is one of the main sources of uncertainty in SFR estimation. Furthermore, conversion from luminosity to SFR is sensitive to the assumed IMF and metallicity. The most common tracers of SFR are UV luminosity (usually at ∼1500–2800 Å), certain recombination lines e.g. Hα and IR continuum emission between ∼3 and 1100 m. Different indicators are sensitive to the star formation on different time-scales: H α line typically probes short ∼10 Myr time-scales while UV and IR provide information on SFR averaged over longer ∼100 Myr time-scales (see e.g. Kennicutt Evans