But Economics Is Not an Experimental Science
指出经济学不是实验科学，自然实验和准实验只是修辞手段，无法回避真正的计量难题；同时批评Angrist和Pischke夸大了某些方法的功效，并讨论了宏观经济学中消除“骗术”的主要进展。
The fact is, economics is not an experimental science and cannot be. “Natural” experiments and “quasi” experiments are not in fact experiments. They are rhetorical devices that are often invoked to avoid having to confront real econometric difficulties. Natural, quasi-, and computational experiments, as well as regression discontinuity design, can all, when well applied, be useful, but none are panaceas. The essay by Angrist and Pischke, in its enthusiasm for some real accomplishments in certain subfields of economics, makes overbroad claims for its favored methodologies. What the essay says about macroeconomics is mainly nonsense. Consequently, I devote the central part of my comment to describing the main developments that have helped take some of the con out of macroeconomics. Recent enthusiasm for single-equation, linear, instrumental variables approaches in applied microeconomics has led many in these fields to avoid undertaking research that would require them to think formally and carefully about the central issues of nonexperimental inference—what I see and many see as the core of econometrics. Providing empirically grounded policy advice necessarily involves confronting these difficult central issues.