You can turn the hypothesis around and measure how much impact an event has on the stock price by determining how much the stock went up or down when the news came out about the event. This hypothesis was used in the legal case of a tax accountant named James Olis
His 24-year sentence stemmed from a calculation of the financial loss caused to investors in Dynegy by Project Alpha, an accounting fraud in which he took part. That financial loss was estimated using the fall in Dynegy's share price on the news that Project Alpha was fraudulent. According to Judge Lake, it was so big that, under sentencing guidelines then in place, Mr Olis had to go to jail for a long time.Any thoughts on the use of this theory to determine damages? The article discusses some of the controversy of using it.
(Source: Freakonomics Blog)