Brownian motion like the bacterial movement observation? Might not be the answer you want with what follows except I'll tell you that if you're trying to do trading math you might chance a solution as to the trader's probability of trade actions except that's a guessing game. A reasoning for derivatives hedging is the sheer amount of capital invested into investment profiles before further trade agreements involving those derivatives ever takes place. They've got money. They use money to make estimated profit from the revenues of portfolios that are proven with statistics to be profitable. That's different than being guaranteed to be profitable.