Thank you for the kind and quick response. I checked the rank plots but didn’t see too much deviation from uniformity, much less than in the example over at rank_plot.
Are there other tricks to narrow down the issue? From looking around, the usual answer seems to be “reparametrize the model”, but I figure as the HSGP is basically a linear model it doesn’t get much simpler…and there is no way to reformulate the non-linear relation…What do you mean by the “(yet)” above? Should I maybe consider a longer tune-in phase?