Bayesian model comparison between models that include different subsets of chemical species. Starting with the reference model (which includes all chemical species), a selected atom or molecule is excluded. If the Bayesian evidence decreases significantly (such that the logarithm of the Bayes factor is much greater than unity), then this chemical species is considered part of the “simplified model” (which contains the minimal set of species required to fit the data). All other chemical species that produce increases in the Bayesian evidence when they are excluded from the retrieval are thus excluded from the simplified model. In each of the panels, the natural logarithm of the Bayes factor (relative to the reference model) is displayed. Retrievals assuming grey (left column) versus non-grey (right panel) clouds are considered separately. The sophistication of the temperature-pressure profile used is based on the findings of Fig. 4.