Research suggests that stock prices may overreact to news announcements and that the pricing errors are corrected only slowly When corporations announce a major change in earnings, a large decline—the stock price may overshoot, and after an initial large decline, it may rise back to more normal levels over a period of several weeks. This violates the efficient market hypothesis, because an investor could earn abnormally high returns, on average, by buying a stock immediately after a poor earnings announcement and then selling it after a couple of weeks when it has risen back to normal levels