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It changes from poker room to poker room.

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The following is from a recent column by Lee Jones in CardPlayer:

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Question: Do you (an online poker site) shuffle a single deck of cards and deal from it, or do you take a random card each time?

Answer: It varies. PokerStars shuffles a 52-card deck, and then starts dealing from the top. Some other sites do that, as well, while other sites choose a card randomly from the deck each time a card is needed. Either approach works just fine as long as you don’t put the cards back in the deck (“without replacement” is the official probability and stats term). And you can bet that the deal is just as random (if not more so) as the shuffle done in a brick-and-mortar cardroom.

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And in spite of paranoia about hacking, I have seen no evidence that it makes a difference which method is used.