eli5 how do casinos determine if someone is counting cards?

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eli5 how do casinos determine if someone is counting cards?

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In some games such as Blackjack, particular playing strategies may indicate that someone is paying close attention to the cards that have been dealt

For example, a player who plays a few hands at minimum bet, and then raising their bet dramatically a few hands into a particular shoe only return to minimum the next time the shoe is shuffled, could be a card counter..

Some playing strategy deviations could also be suspect. For example, splitting a pair of 10s against a weak dealer up card, or taking insurance bets often becomes profitable if the shoe is very rich in 10-valued cards.

The “trick” to counting cards and making money is to vary the bets according to the state of the deck (number of 10-K vs others). Although the casino cannot PROVE it, they don’t have to. They can throw anyone out just on suspicion. As long as there is a somewhat clear and long running winning streak and the player varies their bets from game to game, it can be spotted.

Most blackjack players play with fairly constant stakes, so betting $100 one round, the upping to to $500 a few rounds later only to reduce it to $10 when the deck is shuffled can be a rather obvious sign.

Counting cards is a simple idea: in blackjack, the cards that have already been dealt can’t be dealt again until a reshuffle. Therefore you can determine some information about what cards are coming next. The more high cards that are left, the better for the player (more blackjacks and more dealer busts). If you know this information, you can bet more money when you’re more likely to win, and less money when the game is worse.

Casinos also know how all of this works. They’re also tracking every card in the game to verify it’s running properly. The casino can compare your betting patterns to the known state of the game, and see if you’re betting like a card counter would.

The other factor is that they don’t need to “prove” it – they can throw you out for any reason or no reason. So if they think you might be counting and you’re winning a lot of money, you might be “politely” asked to leave.

Side question: how many decks in a shoe?

Casinos can play blackjack with 5+ decks at a time, so card counting really isn’t that effective anymore