  We provide further evidence that markets trend on the medium term (months)
and mean-revert on the long term (several years). Our results bolster Black's
intuition that prices tend to be off roughly by a factor of 2, and take years
to equilibrate. The story behind these results fits well with the existence of
two types of behaviour in financial markets: "chartists", who act as trend
followers, and "fundamentalists", who set in when the price is clearly out of
line. Mean-reversion is a self-correcting mechanism, tempering (albeit only
weakly) the exuberance of financial markets.
