The concept of “opportunity cost,” as identified by Wieser, is still widely used in modern economic analysis. Eugen von Böhm-Bawerk developed marginal-utility analysis into a theory of price. Böhm-Bawerk is best known, however, for his work on capital and interest, in which he emphasized the role of time in determining the value of goods. He viewed interest as the charge for the use of capital—a compensation to the owner for abstaining from present consumption. The rate of interest was determined by the size of the labour force, the amount of a community's capital, and the possibility of increasing productivity through methods of production. The two leading Austrian economists of the 20th century were Ludwig von Mises and Friedrich A. Hayek. Mises (in the 1920s) and Hayek (in the 1940s) both showed that a complex economy cannot be rationally planned because true market prices are absent. As a result, the information critical for centralized planning cannot be obtained.