Dow Jones (DJIA) P/E Ratio Chart

2011 Dow Jones (DJIA) P/E low is 13.60 ...

If we know historical Dow Jones (DJIA) P/E lows, that will be a good indicator to predict the bottom approximately in future.

Some of the historical figures of P/E at earnings period (This probably doesn't include the P/E lows and highs),

DJIA earnings in prior years:	EPS	Historical GAAP P/E	Historical Earnings Yield
2009 Actual GAAP Earnings (reported Jan 2010)	$624	16.7	5.99%
2008 Actual GAAP Earnings	$661	13.3	7.52%
2006 Actual GAAP Earnings	$720	17.3	5.78%
2005 Actual GAAP Earnings	$476	22.5	4.44%
2004 Actual GAAP Earnings	$592	18.2	5.49%

The average P/E for the Dow Jones Industrial average (DJIA) since 1929 has been 15.5
DJI P/E has gone abnormally high in the past due to near-zero earnings and not due to optimism (1933 P/E 47.3, 1982 P/E 114.4, and 1991 P/E 64.3)

I think its a fair analysis to say Dow Jones P/E of less than 14 are good levels to BUY.