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This seems more like a sinuous function than an exponential one to me. Having more data would likely show a seasonal trend. Interestingly, it starts and finishes with the same value.
What I think would be pretty interesting would be comparing the results of the continents that got glaciers and monsters to the continents that did not, to see what impact it had on retention.
I would agree with this, but unfortunately that would be skewed by the fact that the continents that did not have special rules to them that would possibly affect the results. Beluaterra is an immigration only continent. You have to move a character from somewhere else to there. The other continent that wasn't affected, the Colonies, has only one turn per day, so the activity would significantly be affected by that.
Density doesn't matter with the current estate system. Lords who have an empty estate just lose money so that number of free estate slots in a realm is always tiny and many players per region just lead to people getting shafted in tiny estates.