Major Stock Selloffs Just Aren't What They Used to Be

Stocks Head for Worst Slump Since August

Volatility is back -- at least by this year’s standards. The 0.8 percent retreat in the S&P 500 Index just after noon in New York is what passes for a two-sigma drop in the benchmark gauge.

For calendar year 2016, it would’ve taken a 1.6 percent decline to qualify as a two-sigma move. In 2015, a sell-off of 2 percent was needed for such a change in the standard deviation of the benchmark index.

In English, this is a move you’d expect less than 5 percent of the time.

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