Since things are looking kinda bearish lately, I was thinking about the theory of mean reversion. After doing some searching, I can't find anything that clearly calls out what the mean of the S&P 500 is. I know it's an average of all data sets, but what data points would one use for that average? Yearly, quarterly, weekly, or monthly? Would any time scale return the same results, as long as there was a large enough sample? Also, does anyone think that reversion to the mean is an inevitability?
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