What Is an Asset Price Bubble? An Operational Definition
回顾并分析了资产价格泡沫的现有定义，提出一个操作性定义：当未来某段时期的实际资产回报率偏离预期回报率超过两个标准差时即为泡沫。用此框架检验1929年和1987年股灾，发现它们并非泡沫，而1932年低点反而是“负泡沫”；互联网股票则几乎确定是泡沫。
This paper reviews and analyses the current definitions of bubbles in asset prices. It makes the case that one cannot identify a bubble immediately, but one has to wait a sufficient amount of time to determine whether the previous prices can be justified by subsequent cash flows. The paper proposes an operational definition of a bubble as any time the realised asset return over given future period is more than two standard deviations from its expected return. Using this framework, the paper shows how the great crash of 1929 and 1987—both periods generally characterised as bubbles—prove not to be bubbles but the low point in stock prices in 1932 is a ‘negative bubble.’ The paper then extends this analysis to the internet stocks and concludes that it is virtually certain that it is a bubble.