"Trump tweets a company name and the stock price decreases" is a signal. If you think about trading strategies as collections of consistent signals you can exec

"Trump tweets a company name and the stock price decreases" is a signal. If you think about trading strategies as collections of consistent signals you can execute on, rather than assumptions that require market rationality, it doesn't really matter that a rogue event can cause an unpredictable response. The law of large numbers will generally protect you, given a consistent strategy and enough capital. As an example, a hedge fund that consumes data capable of accurately forecasting earnings in advance can also diversify by consuming social media data. Matt Levine has written about profitable trading strategies developed with sentiment analysis on Trump's tweets. Instead of a hedge fund betting on the stock going up and getting blindsided by a random Trump tweet (or other irrational thing), it will use that as a signal if it can.