Recall Huberman, Gur, and Tomer Regev. Contagious Speculation and a Cure for Cancer: A Nonevent that Made Stock Prices Soar, Journal of Finance, 2001. The paper finds that both ENMDs stock price and biotechnology index increased tremendously following a New York Times article covering the breakthrough of ENMDs cancer research. However, this breakthrough had been already covered by an article in Nature six months before the NY Times article was published. The authors conclude that the stock market violates the efficient market hypothesis because the whole biological index reacts positively to stale information.

6. What is the crucial assumption behind the papers claim that efficient market hypothesis is violated?