If security markets were truly strong-form efficient,one could never earn a realized return on a stock greater than the marginal investor's expected (and required) rate of return on the stock.

In strong-form efficient markets, all information, both public and private, is already reflected in stock prices. However, this does not guarantee that an investor cannot earn a return greater than the marginal investor's expected rate of return, as unexpected events can still cause stock prices to change, providing opportunities for higher returns.