Hat tip to Lux Umbra Dei at TPMCafe for catching this one.

October 2008 was certainly a spectacular month in the stock markets. Large daily changes occurred that surprised most investors. Yet, although many investors had not seen such wild gyrations of stock prices for a long time, there was a general sense that this had happened before.

Those of us who studied modern finance theory, however, were truly astonished by the sheer improbability of the events occurring in the stock markets during that fateful month. One of the basic assumptions used in almost all our finance models is that returns are normally distributed. These models are widely used to price derivatives and other complex financial products. What do these models tell us about the probabilities of the events that occurred in October?