3D PCA coefficients for Log(M_star) (in red), Log(SFR) (in orange), and 12 + log(O/H) (in blue) plotted against the injected noise level σ; different line types correspond to the two different M_star distributions as described in the text and illustrated in the legend. The different curves for the constant M_star case correspond to different lower-mass limits. The true input of 3DPCA²(OH) is shown by the transparent lines, with a thickness of ±0.025 dex. The data points, including error bars that are smaller than the symbols, report the PCA coefficients from the initial σ = 0.15 dex mock sample, but which has been in turn perturbed as described in the text. The aim of this subsequent exercise is to simulate a Monte Carlo perturbation of an observed sample.