Relation between EW uncertainty (or stellar metallicity on the left y-axis) and input spectrum S/N. This is obtained through Monte Carlo simulations where Gaussian noise is added to Starburst99 templates with different metallicities. In this figure, different markers and colors refer to different metallicity indexes used in this work. For this example, we used the template with a metallicity of Z_* = 0.2 × Z_⊙, close to the value of our selected VANDELS galaxies. The vertical green line highlights the S/N range of the stacks that we analyze in this paper, while the smaller vertical gray bars indicate the S/N reached by the stacks in stellar mass from which the mass-metallicity relation is derived in Sect. 3.1.