If my universe of business possibilities was limited, say, to private companies in Omaha, I would, first, try to assess the long-term economic characteristics of each business; second, assess the quality of the people in charge of running it; and, third, try to buy into a few of the best operations at a sensible price. I certainly would not wish to own an equal part of every business in town. Why, then, should Berkshire take a different tack when dealing with the larger universe of public companies? And since finding great businesses and outstanding managers is so difficult, why should we discard proven products? (I was tempted to say "the real thing.") Our motto is: "If at first you do succeed, quit trying."

Warren E. Buffett
http://www.berkshirehathaway.com/letters/1991.html