Parameters and free-riding behavior: multilevel logistic regression analysis with an interaction term between the risk of infection parameter and proportion of others vaccinated.

*P<0.05, **P<0.01, ***P<0.001
aNo formal statistical test was conducted to test if the random effect equals zero.
Observations from Round 2 to Round 24 were used for the analysis. The random effect at the group level was not shown as it was estimated as zero.