Last weekend’s Intelligent Investor column looked at the extreme difficulties of disentangling skill from luck when you are evaluating investment performance. It’s the topic of an excellent new book by Michael Mauboussin and a subject of endless fascination – and frustration – to investors.

We tend to think of the greatest investors – say, Peter Lynch, George Soros, John Templeton, Warren Buffett, Benjamin Graham – as being mostly or entirely skillful.

Graham, of course, was the founder of security analysis as a profession, Buffett’s professor and first boss, and the author of the classic book The Intelligent Investor. He is universally regarded as one of the best investors of the 20th century.

But Graham, who outperformed the stock market by an annual average of at least 2.5 percentage points for more than two decades, coyly admitted that much of his remarkable track record may have been due to luck.

In the Postscript chapter of The Intelligent Investor, Graham described “two partners” of an investment firm who put roughly 20% of the assets they managed into a single stock – a highly unusual departure for the conservative managers, who normally diversified widely and seldom invested more than 5% or so in any one holding. (more…)