Is a 4% Down Day a Black Swan?

On February 5, the SP500 experienced a drop of 4% in a day. We ask ourselves the question:  is a one-day 4% drop a common occurrence? The table below shows the number of 4% (or more) down days since 1970.

	4% down	4% down and bullish
From 1970	40	5

 

On average, a 4% down day occurred each 1.2 years, which is probably not a rare occurrence.

We next counted the number of days when the SP500 dropped 4% or more during a bull market. We defined the bull market as price > 200-Day simple moving average.  Since 1970 there have been 5 occurrences, i.e. on average once every 10 years. We don’t know whether this qualifies as a black swan event, but a drop of more than 4% during a bull market is indeed very rare.

The table below shows the dates of such  occurrences. It’s interesting to note that before the February 5 event, the last two 4% drops when price> 200-day SMA occurred around the dot-com period.

Date	%change
September 11, 1986	-4.8
October 13, 1989	-6.1
October 27, 1997	-6.9
April 14, 2000	-5.8
February 5, 2018	-4.1

 

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