Precedents and Prospects for Randomized Experiments

When families move to low-poverty neighborhoods, their teenage children are less likely to commit crimes (Ludwig, Hirschfield, and Duncan 2001). Couples therapy and family therapy are equally effective at improving marital relationships (Shadish et al. 1995). Increasing welfare benefit amounts by 10 percent discourages 1 percent of low-income parents from working (Burtless 1987). Each of these statements answers a question about the effect, or impact, of a social policy or intervention on people’s behavior. Does helping low-income families move to low-poverty neighborhoods affect their children’s development? Does couples counseling bring more benefits than family therapy? What proportion of low-income parents