In The Shiller Ten-Year P/E Ratio I wrote, “Sometimes during recessions a company’s earnings can be zero or negative over several quarters. When the trailing 12 months of earnings approaches zero, the P/E ttm approaches infinity. This divide-by-zero error often happens when the stock is about to recover.”

Charlie Tian’s GuruFocus.com has a nice article on Shiller P/E – A Better Measurement of Market Valuation in which they write:

The regular P/E uses the ratio of the S&P 500 index over the trailing-12-month earnings of S&P 500 companies. During economic expansions, companies have high profit margins and earnings. The P/E ratio then becomes artificially low due to higher earnings. During recessions, profit margins are low and earnings are low. Then the regular P/E ratio becomes higher. It is most obvious in the chart below:

The highest peak for the regular P/E was 123 in the first quarter of 2009. By then the S&P 500 had crashed more than 50% from its peak in 2007. The P/E was high because earnings were depressed. With the P/E at 123 in the first quarter of 2009, much higher than the historical mean of 15, it was the best time in recent history to buy stocks. On the other hand, the Shiller P/E was at 13.3, its lowest level in decades, correctly indicating a better time to buy stocks.

And if you would like to learn how to apply these numbers:

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