Quote from Lawrence Chan:
There is a reason why the S&P can pop back up that fast since mid-2005.
As I recall, S&P changed its calculation method of the S&P index in mid-2005 to a modified version of the true capitalization weighting method. That means, it is going to weight each stock based on flow, not the true # of outstanding shares.
The old high in S&P 500 has to be recaculated using the new method to give a proper comparison.
What I implied is that if volatility is going to increase like the 98-00 blow off phrase, and that we are at the starting point only (200 points S&P higher), the new calculation method can have an interesting effect that magnify/dampen the volatility.