As pointed out by Warren Buffett , the percentage of total market cap (TMC) relative to the U.S. GNP is "probably the best single measure of where valuations stand at any given moment." Given a 2.6% 10-year Treasury bond yield, investors may be comfortable accepting weak prospective long-term returns for the S&P 500 - we estimate about 2.8% annually over the coming decade. In all of the three approaches discussed above, the fluctuations of profit margin are eliminated by using GDP, the average of trailing 10-year inflation-adjusted earnings and peak-P/E, revenue or book value, etc.