The crash was a grotesquely unlikely event on the log-normal model of stock-price movements underpinning that theory. In terms of the log-normal model, the fall on October 19 of "the two month S&P 500 futures price ... is a -27 standard deviation event with probability 10-160" (Jackwerth and Rubinstein 1996, pp. 1611-1612).

Donald MacKenzie, An Engine, Not a Camera, loc. 845