George Box has said "The only way to find out what will happen when a complex system is disturbed is to disturb the system, not merely to observe it passively." These words of caution about "natural experiments" are uncomfortably strong. Yet in today's world we see no alternative to accepting them as, if anything, too weak.

- by Fred Mosteller and John Tukey, 1977-

Kyung Min Lee

Schar School of Policy and Government

George Mason University

3351 Fairfax Drive, Arlington, VA 22201