I think, the period that u have mentioned is related to the peak of bull market.
During bear market period of 2002 to 2003, The PE range was from 10.5 to 19.4.
At the end of 2003, when bull market was already on, we crossed the PE level of 20.
As per my analysis, PE = 20 falls at the higher range of 1 Standard deviation. I.e. market value above this is an exception / abnormal.
Certainly market valuation is stretched.
Moreover, this PE is based on previous earnings. IMO, forward PE will still be higher then 20 cause most of the companies have given lower earning guideline.
Anyway, this number just helps in drawing the big picture and has very little relevance in day/short term trading.
Happy Trading.