The ratio was published by Shiller in his book, Irrational Exuberance. It is a cyclically adjusted P-E ratio (CAPE ratio).
It takes 10 year earnings into account and adjusts it for inflation. For S&P 500, the ratio is updated regularly on Yale website.
According to http://multpl.com, the current shiller P-E ratio is 23.56 (18 March 2013).
The highest was 44.20 reached in December 1999 and the lowest was 4.78 in December 1920. The median figure is 15.88.
