Looking down my blogroll, I see that the wise and thorough 'Woodshedder' has beat me to the punch with an excellent post on this topic, so I will keep this brief.
As you will note below, the odds of a next-higher day fall precipitously compared to average for each successive day that the S&P500 RSI exceeds 90. We had had readings over 90 for the last four days; nearly five by a hair counting December 30th's 89+ reading. As has been discussed extensively throughout the financial blog-o-sphere (most notably over at MarketSci), the mean-reversion versus trend tendency has strengthened since the late-nineties.
Postscript: The pending ETF Rewind presents a five-day history of short-term RSIs for nearly 170 ETFs across nine major asset classes. How great is that!
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