[1] Univ Michigan, Sch Informat, Ctr Study Complex Syst, Ann Arbor, MI 48109 USA
[2] Fed Reserve Board, Div Res & Stat, Washington, DC USA
[3] Amer Univ, Kogod Sch Business, Washington, DC 20016 USA
[4] Imperial Coll, Ctr Global Finance & Technol, London, England