Two research tools that have emerged over the past twenty-five years have greatly expanded economists’ repertoire for learning about the world. The first is the use of randomized control trial experiments, long used in other scientific fields such as medicine. The typical study investigates what happens when some people receive some “treatment” of interest. The second approach is to use either naturally occurring experiments (such as when some people are enrolled in a program and others are not) or clever econometrics techniques that manage to detect the impact of treatments even though no one
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