Before revision:  However, the behaviour of the model is in general not tractable for large networks ; therefore, mean field theories are frequently used to approximate its statistical properties. Many variants of the classical naive and TAP (i.e., second-order) mean field approximations have been proposed , each of which makes unique assumptions about the temporal evolution of the system's correlation structure .
After revision:  However, the behaviour of the model is in general not tractable for large networks , many mean-field methods have been proposed , each of which makes unique assumptions about the temporal evolution of the system's correlation structure .