Blackjack players at a casino will spend a lot of time deciding how much to bet on each hand. At least the serious ones do. Often, they even have a system. Traders may be surprised to realize they might not be as well prepared as savvy card players.
Traders buy and sell for a variety of reasons. Some traders will make decisions based upon news headlines or gut feelings. One may read that gasoline prices are at record levels and decide they have to go down, while with equal certainty another trader decides the news justifies even higher prices. These strategies rely on luck, rather than mathematical probabilities, to succeed.