Investors who focus on currencies, bonds, and stock markets generally assume a normal distribution of price changes: values jiggle up and down, but extreme moves are unusual. Of course, extreme moves are possible, as financial crashes show. But between 1985 and 2015, the S&P 500 stock index budged less than 3 percent from its starting point on 7,663 out of 7,817 days; in other words, for fully 98 percent of the time, the market is remarkably stable.
