Research from Bloomberg shows:

  • “Since 1951 there have been 57 four-standard-deviation-plus trading days for the S&P 500. Four of them have been since August 4.
  • Such events statistically are supposed to happen only once every 100 years.
  • The largest move in the recent series came on August 8, when the S&P 500 lost 6.7 percent. That’s the equivalent of a 6.2-standard-deviation move, which should occur only once every 2 million years.
  • The largest single move since 1951 was on Oct. 19, 1987, when the S&P lost 20.5 percent in a single session, the equivalent of 20 standard deviations.”