Background: you can use factor analysis on mental illness to get a general psychopathology factor, like the g factor in intelligence but instead a general mental illness factor.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4209412/

Intent: I am interested in using a forecast aggregation platform to predict whether we will discover a Flynn effect like trend in p factor, either up or down. https://www.metaculus.com/questions/

Caveat: Flynn may be hollow for g, and in theory something like that could happen with a p factor trend too. That's fine. Our forecast question can be agnostic about the nature of the trend, in order to be reliably resolvable without too much administrative effort.

Specific need: what simple metric, or criteria wording, could be shown to the forecasters, such that they could predict something that will reliably resolve? In forecasting, the event description must be unambiguous. This is harder than people new to forecasting tend to expect. One thing that might affect our criterion choice is how we think future researchers will measure such a thing, as piggybacking off already-reported metrics can make forecast question-writing easier.

E.g. suppose if we had the following wording: "Will there be data to support a trend in the p factor, at least equivalent to an average 2 IQ point-per-decade change, either up or down, during any 10-year period before 2040?" Would it be easy for people not working in any related field to kind of just, do a cursory search around 2040 (or every few years) and clearly tell if such a thing has happened? If not, what would be a better way to word it?

Any input appreciated!