Van de Bunt, G.G., M.A.J. van Duijn, and T.A.B. Snijders (1999).
Friendship networks through time: An actor-oriented statistical network model.
Computational and Mathematical Organization Theory, 5, 167-192.
Also see http://www.stats.ox.ac.uk/~snijders/siena/vdBunt_data.htm.