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When Buffett was asked by business students in 2008 about his views on portfolio diversification and position sizing, he responded that he had “two views on diversification:”13 If you are a professional and have confidence, then I would advocate lots of concentration. For everyone else, if it’s not your game, participate in total diversification. If it’s your game, diversification doesn’t make sense. It’s crazy to put money in your twentieth choice rather than your first choice. . . . [Berkshire vice-chairman] Charlie [Munger] and I operated mostly with five positions. If I were running $50, $100, $200 million, I would have 80 percent in five positions, with 25 percent for the largest. In 1964 I found a position I was willing to go heavier into, up to 40 percent. I told investors they could pull their money out. None did. The position was American Express after the Salad Oil Scandal.
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Allen C. Benello (Concentrated Investing: Strategies of the World's Greatest Concentrated Value Investors)