In the great bull market of the 1920s relatively little distinction was drawn between industry leaders and other listed issues, provided the latter were of respectable size. The public felt that a middle-sized company was strong enough to weather storms and that it had a better chance for really spectacular expansion than one that was already of major dimensions. The depression years 1931–32, however, had a particularly devastating impact on the companies below the first rank either in size or in inherent stability. As a result of that experience investors have since developed a pronounced preference for industry leaders and a corresponding lack of interest most of the time in the ordinary company of secondary importance. This has meant that the latter group have usually sold at much lower prices in relation to earnings and assets than have the former. It has meant further that in many instances the price has fallen so low as to establish the issue in the bargain class.
―The Intelligent Investor, Chapter 7