An unfair coin (potentially EXTREMELY unfair) will be flipped fifty times. Your goal is to correctly predict as many of these flips as possible, by deducing the nature of the unfairness as quickly as possible.

[Predict Heads] [Predict Tails]

You can play this (in-browser, very short) game here; the rule governing the unfairness is automatically revealed after flip 50. Followups with different governing rules are here, here, here, here and here.

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14 comments, sorted by Click to highlight new comments since: Today at 3:49 AM

Note that this is NOT an "unfair" or even "EXTREMELY unfair" coin. It's not a fixed bias or even a wear pattern or other understandably-moving probability.  There is no possible way for a real coin to have that distribution.

It would be a more interesting prediction challenge if the range of posssible patterns were known.  "I've coded something that doesn't make sense; go!" is not that helpful.  Or even just say "predict the output of this program, which is absolutely not a coin".

Just like life, then!

There is no possible way for a real coin to have that distribution.


Unless the person throwing read Jaynes:

a person familiar with the laws of mechanics can toss a biased coin so that it will produce predominantly either heads or tails, at will. [...] From the fact that we have seen a strong preponderance of heads, we cannot conclude legitimately that the coin is biased; it may be biased, or it may have been tossed in a way that systematically favors heads. Likewise, from the fact that we have seen equal numbers of heads and tails, we cannot conclude legitimately that the coin is ‘honest’. It may be honest, or it may have been tossed in a way that nullifies the effect of its bias.

More on how:

An important feature of this tumbling motion is conservation of angular momentum;
during its flight the angular momentum of the coin maintains a fixed direction in space (but
the angular velocity does not; and so the tumbling may appear chaotic to the eye). Let us
denote this fixed direction by the unit vector n; it can be any direction you choose, and it
is determined by the particular kind of twist you give the coin at the instant of launching.
Whether the coin is biased or not, it will show the same face throughout the motion if viewed
from this direction (unless, of course, n is exactly perpendicular to the axis of the coin, in
which case it shows no face at all).
Therefore, in order to know which face will be uppermost in your hand, you have only
to carry out the following procedure. Denote by k a unit vector passing through the coin
along its axis, with its point on the ‘heads’ side. Now toss the coin with a twist so that k and
n make an acute angle, then catch it with your palm held flat, in a plane normal to n. On
successive tosses, you can let the direction of n, the magnitude of the angular momentum,
and the angle between n and k, vary widely; the tumbling motion will then appear entirely
different to the eye on different tosses, and it would require almost superhuman powers of
observation to discover your strategy.
Thus, anyone familiar with the law of conservation of angular momentum can, after some
practice, cheat at the usual coin-toss game and call his shots with 100% accuracy. You can
obtain any frequency of heads you want – and the bias of the coin has no influence at all
on the results!

I had an expectation that it could be a very weird type of bias given that the text says to predict the nature of the unfairness, not just a direction or something like that. I agree that calling it a "coin" is quite misleading though.

This would be more to my taste (I can't speak for anyone else's) if we were told more about the space of possible unfairnesses. In particular, it wasn't clear to me whether

the coin flips are allowed to depend on our predictions (and, if so, which predictions)

nor whether

we were actually looking for a genuinely probabilistic rule (in which case it would have to be a very simple one for there to be any chance of guessing it) or for a deterministic one, possibly depending on the predictions (in which case it might be more complicated).

Again, I don't claim that my taste is anyone else's; but my reaction to the extreme open-endedness is along the lines of "this could be practically anything, and some varieties of thing-it-could-be are obviously not deducible with any confidence from 50 bits of information, and this has probably been designed so that it's solvable if you correctly guess what space of possibilities the creator had in mind but I don't feel like trying to read his mind".

The spoiler-blocks above aren't very spoilery since they are just asking questions. But for the benefit of anyone who feels the same way as I do, here are what I now believe to be the answers to those questions:

The coin does not know anything about your predictions; you are trying to model a thing that autonomously emits coin flips, not something that actively responds to your probing. Unsurprisingly-given-that, the coin is (at least potentially) probabilistic.

You link to index C twice, rather than linking to index D. (And index D was such an interesting one too.)

Anyways, this is very fun. I made a couple (fairly easy) coins of my own, should be runnable by pasting into the console in your browser's dev tools (while you're on the fifty flips page, of course):


>You link to index C twice, rather than linking to index D. 

Whoops! Fixed now, thank you.

I assumed that if you are flipping a coin, trials would be independent events and each flip would have a fixed rule (which is what happens when you flip a single coin). Instead, the coin had a different rule for odd and even numbered flips. I think that the language of the website should be amended to reflect this.

Do you mean the coin has a fixed probability of heads for each of the 50 flips (perhaps drawn from some distribution at the beginning of the game)?

Or can the probability change for each flip based on some more complicated rule?

The coin does not have a fixed probability on each flip.

Boy howdy was I having trouble with spoiler text on markdown.

I liked this! The game was plenty interesting and reasonably introduced. It's a fun twist on induction games with the addition of reasoning over uncertainty rather than exactly guessing a rule, though it does have the downside the relatively small number of samples can make the payoff dominated by randomness.

To offer one small piece of constructive advice on the execution, I did wish the flip history autoscrolled to the newest entry.

I got alternating THTHTHTHTH... for the first 28 flips, which I would have thought would be very unlikely on priors for the 80% rule. Are you sure that's an accurate description of the rule? It doesn't change halfway through?

It's pretty unlikely, but not extraordinarily unlikely. I wouldn't be surprised to learn that the site has been run a few hundred times, which would make it about 50-50 for someone to see that.