Buffett's Best Valuation Measure Is Flashing Danger

In March of 2009, the S&P 500 bottomed at 666. Today it's trading near 2200, which equates to a 226 percent increase in the large-cap stock index over the last seven-and-a-half years. There is no question that this bull market has been strong and long-lasting. Over the past few months, some experts have argued the current trend is long in the tooth and that stock valuations (as measured by metrics, such as the Shiller P/E) are stretched.