Comment Re:Without SEEING the formula, it's rather difficu (Score 2, Informative) 160
They measure cumulative size distribution (how many groups of size >= N) and churn (how many people leave the group for another one in a given period).
They are able to come up with a simple mathematical model for the behaviour of players (essentially: recruit people with diverse attributes/skills) that reproduces the observed data extremely well. And they also show that the alternative 'kinship' model (recruit people with similar attributes/skills) fails to reproduce the observed data.
I would say that their model does quite a good job at modeling some rather nontrivial data.