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When the market is up 4%, the next day it's a coin flip

Traders work on the floor of the New York Stock Exchange.
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Traders work on the floor of the New York Stock Exchange.

On Wednesday, the S&P 500 index was up 3.9 percent. So we wanted to see what happens the day after a move that large.

Since 1978 the SPX has seen a gain of 3.5-4.5 percent on 38 different occasions; Wednesday was the 39th time it happened.

In the previous 38 circumstances, the market was basically a coin flip the next day. Here are the stats:

+4.03% best day

-5.27% worst day

-0.10% mean day

-0.10% median day

19 positive days

19 negative days

Notice that the average performance is basically ZERO. And there are exactly the same number of positive days as negative days.

So tomorrow could literally be a coin flip. Anything is possible.

Read MoreWe're up because we broke through this key indicator