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Comment by lumost | Hacker News Reader

From a physics stand point, coin tossing is similar to knife throwing or axe throwing. It's completely within human capability to intentionally or unintentionally time the toss with some degree of accuracy. I doubt anyone can be "good" at this (otherwise it would be a great grifting trick), but surely this creates at least a marginal bias in the data at a high enough scale.

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In roulette this is known as the "dealers signature" where a bored zoned out dealer can sometimes hit the same region/sector of the wheel on consecutive spins.

I've seen this happen (or at least appear to happen) in real life where an obviously bored dealer was consistently hitting the same 1/3'd of the wheel and players were taking advantage of it. After a while a suit shows up and starts giving heat not to the players but to the dealer. Chatting them up with nonsense conversation to distract them out of their zone. This wasn't a pit boss/supervisor but casino security - guys that emerge from back rooms to give heat to card counters in blackjack.

The distraction worked: the now very awake and nervous dealer was no longer hitting similar areas and the players moved on to other tables.

When I was around 15 I tried to master the controlled coin flip, having been exposed to the idea in one of the Stainless Steel Rat books by Harry Harrison. I reached around 80% success flipping for heads, so long as I used the catch-and-show method. It was easier with heavier and bigger coins, tougher with lighter and smaller ones. Letting the coin drop the the ground and bounce took me back to 50%.

Satisfied that it could be done, I moved on - never used the skill, except as a party trick.

This is why the casino people get upset when you roll the dice at a craps table and continually avoid hitting the back wall. The rubber pyramids lining the wall are critical to ensuring they maintain their edge :)

Same here. Long ago I tried to generate an encryption key by flipping a coin. After getting a rhythm and flipping heads 20 consecutive times I gave up.

You could still XOR longer runs together, reducing the bitrate but mostly eliminating the bias.

There is a protocol for this... you flip the coin twice. HH or TT you ignore, HT or TH you keep (and treat as H or T). This eliminates any bias towards H or T that the coin or flipping procedure may have.

From what I've heard, Perci Diaconis (one of the authors of the original paper) actually could do this. He was a magician before he became a mathematician, and a lot of his early mathematics work focused on math relating to the magic tricks he used to do

Art Benjamin at Harvey Mudd College is another magician/mathematician. The two skills line up pretty well, it seems. When I got my math teaching credential there were two people in my cohort who did magic tricks as well as at least one teacher at the school where I did my student teaching.

Even that poses a fun test. What flipping method is needed to ensure some level of randomness.

I love that idea of a grifter being able to flip a coin as they choose.

Probably a device that would uniformly toss the coin with regards to force applied at X number of different strengths.

Or have both people flip a coin secretly, then reveal the coins and XOR the results :D

Maybe letting the coin fall on a hard surface, so that it has a chance to bounce/spin randomly before falling flat (or staying on its side!)

People can definitely get "good" at coin tossing. It isn't even too difficult a trick to master. It's not really possible to use it in any kind of grift scenario though, because there aren't any real world cases where you can win money (or gain any advantage) just by tossing a coin a certain way. Rolling dice has a lot more potential, but those are in turn heavily monitored, e.g. in casino settings.

> there aren't any real world cases where you can win money (or gain any advantage) just by tossing a coin a certain way.

Paying off a referee to have the coin turn up heads in overtime in a football game seems like it may net you some profit.

If you can pay off a referee then the coin toss would be the least effective way to do it. A single bad call would gain you a much bigger advantage.

It has far more plausible deniability, however.

Maybe. It only takes one bad call at the right time to throw a game. If the ref is consistently bad and missing call for both sides what is one more bad call?

I don't care much about bad refs so long as they are consistently bad. However a perfect ref who missing one critical call is a lot more suspicious. Of course I prefer a great ref, but I can work with a fairly bad ref.

I believe NFL coin tosses let the coin hit the ground. All methods I know of biasing a coin toss involve catching the coin. As long as the coin hits the ground and bounces at least once, a lot of randomness is re-introduced.

Antidotally I can confirm that it is possible toss a coin like that. It helps have a larger coin and to impart it with relatively little rotation.

I can consistently flip a US quarter and get the desired outcome. I have not measured accuracy but is definitely 80% or more. This can be with a quarter I just picked up at a store… no special wear.

Forcing a coin toss of the sort described in the article is well within reach of the interested stage magician.

Forcing a coin toss of the kind used in sport, where the coin hits the ground, is more difficult.

You don’t have to be a magician. I was impressing other kids doing exactly this, you just toss so the coin goes high but turns slowly and catch it in your hand rather than let if fall to the ground. After not so many tries you get a pretty good intuition on how you have to throw to get the desired result.

It works the other way around, this kind of trick is (stage) magic so if you're doing it, hey presto, you're a magician.

There's also a particular method to get the coin in a tilted spin that looks (and sounds) _remarkably_ like a real flip, and is I think going to be more repeatable than what you're describing.

It wouldn't be. People who fall for shell games have to accept that it's fishy and just want to show they are better.

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