Into December of 1919, the CAPE was at an all-time low. With so much focus on 1919 going into the COVID-19 pandemic among market commentary, the comparison that this chart allows to make is 1919 to 2019, with a suggestion of downtrend.
If the SPX indeed returns to multiples in 1919, that would put a market valuation of the SPX at around $500 according to the CAPE ratio.
Comment: In 1998, Robert Shiller--the Yale economist and Nobel Prize winner-- formalized that idea in a paper, "Valuation Ratios and the Long-Run Stock Market Outlook" and a book, "Irrational Exuberance." The latter made Shiller something of an economic prophet. In his book, which came out shortly before the dotcom crash, he warned that stocks were overvalued.
What is the CAPE ratio? It describes the over 10 years, rather than on a particular date. Called the Shiller P/E, it is calculated by dividing the price of a stock by its average over the past 10 years, adjusted for . It can also be used on an index such as the S&P 500 .