From Jack Hough for The Wall Street Journal, here:
The plain old trailing P/E—the one that suggests stock prices are normal—is at least based on known earnings, so it's a good place to start in deciding whether stocks are affordable. But it has a flaw: If the past year's earnings were unsustainably high, today's P/E might be deceivingly low.
One fix is to take an average of many years of trailing earnings. Yale economist Robert Shiller advocates using 10 years, adjusted for inflation. His "cyclically adjusted P/E" is 20.8, versus an average since 1881 of 16.4.
Mr. Shiller's measure suggests stocks are a bit pricey, and another signal agrees. According to government statistics, after-tax corporate earnings are near a record high relative to worker wages. In the past, similar readings have foretold sharp earnings drops.
Today, that scenario isn't certain, because companies make more of their money overseas than in the past. "The government data might be a warning," says Mr. Shiller.
So it is time to be cautious, but not necessarily to flee stocks. U.S. ones are priced to deliver long-term inflation-adjusted returns of 4 percent a year, including dividends, reckons Mr. Shiller, about double what the 10-year Treasury bond pays.
