A positive difference between marginal product and wages is usually said to be evidence of monopsony power; just as the ratio of product price to marginal cost has been suggested as a measure of monopoly power, so has the ratio of marginal product to wages been suggested as a measure of monopsony power. But specific training would also make this ratio greater than one. Does the difference between the marginal product and the earnings of major-league baseball players, for example, measure monopsony power or the return on a team's investment? Since teams do spend a great deal on developing players, some and perhaps most of the difference must be considered a return on investment (even if there were no uncertainty about the abilities of different players).