Sorry to break it to you, card sharks, but your shuffling skills probably suck.
That's the takeaway from a fascinating new video from YouTube's Numberphile series (above). As the video shows, some methods of shuffling are more reliable, and more efficient, than others (scroll down for GIFs) -- and in any case you're probably not doing what it takes to get a random distribution of cards.
According to the video, the familiar "riffle method" of shuffling trumps all the others. It involves holding half of the deck in each hand and then using the thumbs to alternate the cards. But you've got to shuffle seven times to get the job done.
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"If you shuffle fewer times, if you shuffle five or six times, somebody can really make money against you in a card guessing experiment," Dr. Persi Diaconis, a professor of statistics and mathematics at Stanford University, says in the video. "If you shuffle 10 or 11 times, it's not worth the wear and tear on your shoe leather standing there in the casino if you're counting cards. It's just as close to random as it could be."
If you use what Diaconis calls the "smooshing method" (below), you should probably mix the cards for about a minute.
And if some joker decides to use the "overhand method" (below) he/she will be shuffling for... a very long while.
That's the deal!