Such precepts, however, don't really explain how Mr. Buffett outperformed the market; if they did, anyone who had read his annual letters could have done it. Mr. Mehta doesn't explain how he did it, either. Instead, Mr. Mehta points out how unusual Mr. Buffett really is. From a statistical standpoint, he is an anomaly.
Consider this, gleaned from Mr. Buffett's own data on the second page of the Berkshire annual report: From the beginning of 1965 through the end of 2013, he outperformed his own chosen benchmark — the S.&P. 500-stock index, including dividends — by 9.9 percentage points, annualized.
How rare is that? According to Mr. Mehta's analysis, it puts Mr. Buffett in a vanishingly small class, comprising far less than 1 percent of the population of investors. This is the tiny group that is statistically likely to have been able to beat the stock market through that elusive alpha — skill of some sort, rather than just chance — over a period of 45 to 50 years.
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The flip side of Mr. Mehta's finding is also worth considering. A vast majority of individuals, including most people now working in finance, do not have alpha, Mr. Mehta says. It doesn't matter whether they have studied finance or have prodigious math skills; the statistics show that they are unlikely to have the ability to beat the market.
That has a serious implication for individual investors, he says: True investing skill is so rare that the rest of us shouldn't even try to emulate those who have it. In addition, he says, we probably shouldn't bother trying to hire the few outperformers to invest our money. Why? Because we aren't likely to be able to identify them. Their talents aren't always on public display, and there may be only a few thousand of them in the entire United States.
Mr. Buffett's talents are widely known. But despite his celebrated past performance, his returns since the beginning of 2009 have been disappointing.
In four of the last five calendar years, he has underperformed his own benchmark, the S.&P. 500 with dividends, often by significant margins. (In 2011, his return of 4.6 percent beat the benchmark by 2.6 percentage points.) In addition, data provided by Morningstar shows that he underperformed the average stock mutual fund investor in four of the five years.)
By contrast, in the previous decades, he had underperformed the S.&P. only six times. Mr. Mehta said his calculations showed that given such a long period of outperformance, there is only a 3 percent chance that the recent stretch of underperformance was a matter of bad luck.
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What happened? We don't really know, and Mr. Buffett declined to comment for this column. In a section of the annual report, Mr. Buffett notes that because he judges investments based on what he considers to be their intrinsic value, Berkshire may well underperform in periods when stocks rise rapidly. He also says that as Berkshire has grown, he has increasingly been buying entire operating companies, as opposed to investing in shares of publicly traded companies, and, as a result, the long-term strength of his investments may not be fully reflected in his annual data.
Mr. Mehta won't hazard a guess, but he does compare Mr. Buffett to Michael Jordan, the basketball star. "There were essentially two careers," Mr. Mehta said. "In the first, he was a superstar. And in the second, late in his career, he just wasn't one anymore."
Will Mr. Buffett return to form and trounce the market again? He might or he might not. But Mr. Mehta says that most people will be better off with a draw: anyone can match the market with an index fund.
—By Jeff Sommer of The New York Times