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The Russell 2000 is up nine days in a row: A coin flip can do that

Through the close of trading Wednesday, the Russell 2000 has been up nine days in a row. If it can hold on to its midday Thursday gains, that would be 10 days in a row. That might seem like a rare feat, but it's happened 19 times since the late 1980s.

Whether or not the small-cap index closes up Thursday or falls to start a new down streak, basic probability tells us that we will get this kind of streak every once in a while. It's like the illusion of a "hot hand" in basketball: Even a totally average player will have a long scoring streak on occasion.

In the case of the stock market, we can show that its streaks aren't too far off from flipping a coin. When we run a simulated coin flip thousands of times, we get a very similar pattern of "up" streaks. The big-cap S&P 500 is nearly identical to our coin flips. The Russell 2000 is made up of small-cap stocks and is more momentum based, so we see a few more long streaks than you'd expect at random.

Since 1987, there have been about 7,300 trading days. In that time, the Russell has hit a streak of 10 consecutive positive days 19 times. Most of those incidents happened in the 1990s, and they have become a lot more rare since. The most recent 10-day streak in small caps was in March 2013.

Contrast that to the S&P 500, which has only done that four times.

If we flipped a coin 7,300 times, with each flip representing one day in the stock market, we get pretty similar results.

Look at the chart below. The number of streaks at any given length — in either direction — is effectively the same for a coin flip as it is for the stock market.

To be clear, the market isn't exactly a 50-50 proposition, like flipping a fair coin would be. Our data shows the Russell 2000 is up on roughly 54 percent of days. The S&P 500 rises about 53 percent of the time. But just using the 50-50 assumption to compare our theoretical coin, the market shows that these occasional long streaks are not far off from random chance.