For decades there was this debate about Medicaid, the health insurance program for the poor. The debate was perennial and unresolvable. You couldn't simply compare people with Medicaid to those without, because the two groups had different characteristics. Then, a few years back, Oregon announced that it had 10,000 new slots in its Medicaid program. So the state held a lottery. The state of Oregon didn't mean to be creating the perfect experiment to test whether Medicaid actually helps people. But that's what it was doing.