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If you've had a good year so far in the stock market, chances are you had a pretty bad day in the market today. On the surface, today's action wasn't awful. The S&P 500 (SPY) was down 0.5% while the Russell 1,000 was down 0.7%. Within the Russell 1,000, the average stock was down even less at -0.33%.

So what's the big deal? Well, what made today noteworthy was the near-perfect performance distribution based on how stocks had performed so far this year heading into today. In the chart below, we've broken up the Russell 1,000 into deciles (10 groups of 100 stocks each) based on YTD stock performance through last Friday. Decile 1 contains the 100 best-performing stocks YTD, decile 2 contains the next best 100, and so on until you get to decile 10, which contains the year's bottom 10% of performers. In the chart, we show the average performance today of the stocks in each decile. As shown, the decile of the best-performing stocks YTD through last Friday was down 2.9% today, while the decile of the worst-performing stocks YTD was up 2%! As you work your way down from the best decile to the worst, today's performance gets better and better.

At the very top end of the YTD performance spectrum are the 26 stocks in the Russell 1,000 that entered today up more than 100% on the year. As shown below, these stocks fell more than 5% on average today, and every single one was down!
 

On the flip side are the 25 worst-performing stocks YTD coming into today. These 25 stocks were down an average of more than 50% on the year through last Friday, but they averaged a gain of more than 3% today!

Rarely do we see such uniform performance in one direction based on prior performance in the opposite direction, but that's exactly what we saw today. Investors were selling winners and buying losers to quite an extreme degree.

Given this type of action, we wouldn't make any rash long-term investment decisions based on one day's moves. Sometimes it's just opposite day in the market where we see downside mean reversion from winners and upside mean reversion from losers.

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