From the 1978 Berkshire Hathaway shareholders letter:
http://www.berkshirehathaway.com/letters/1978.html
Quote:Our policy is to concentrate holdings. We try to avoid buying a little of this or that when we are only lukewarm about the business or its price. When we are convinced as to attractiveness, we believe in buying worthwhile amounts.
http://www.berkshirehathaway.com/letters/1978.html