The problem is that there is no certain way to explain how people will react to such a policy change, to any signs of price overheating or decline, or to other news stories that might be spun as somehow important. We simply do not have much well-documented history of big financial crises to examine, leaving econometricians vulnerable to serious error, despite studying time series that are typically no more than a few decades long.
问题在于没有确定的方法能够解释人们会如何对此类政策变化做出反应，对任何价格过热或下跌的信号做出反应，或对其他可能具有重要性的报道做出反应。我们缺乏大型金融危机的详细历史记录可供考察，这导致计量经济学家很容易犯下严重错误，尽管他们所研究的时间序列通常不会超过几十年跨度。