A Monthlong Walk on the Wildest Side of the Stock Market

One volatility measure, shown in the accompanying charts, is the number of days in which an index closes up or down at least 4 percent.

In normal times, the market goes years without having even one such day. There were none, for instance, from 2003 through 2007. There were three such days throughout the 1950s and two in the 1960s.

In October, there were nine such days.