# FeaturedI The Sleeping Beauty Problem: Any halfers here?

## What is Sleeping Beauty's credence now for the proposition that the coin landed heads?

12 vote(s)
33.3%

11 vote(s)
30.6%

13 vote(s)
36.1%
1. Jun 2, 2017

### Demystifier

The sleeping beauty problem is a well known problem in probability theory, see e.g.
https://en.wikipedia.org/wiki/Sleeping_Beauty_problem
http://allendowney.blogspot.hr/2015/06/the-sleeping-beauty-problem.html
https://www.quantamagazine.org/solution-sleeping-beautys-dilemma-20160129

Allegedly, there are many "thirders" who think that the correct answer is 1/3, but also many "halfers" who think that the correct answer is 1/2. For me, it is quite obvious that the correct answer is 1/3. Is there anybody here who is convinced that the correct answer is 1/2? If you are one of them, what is your argument for 1/2?

2. Jun 2, 2017

### Staff: Mentor

What is your argument of a third? She will not be interviewed on Wednesday, which leaves two possible days.
As the coin decides on which day(s) she will be awakened, her a priori probability for heads (= Monday only) is .5.
Now the question is, whether her a posteriori probability changed during the experiment. However, due to her amnesia, there is no way for her to gain additional information, except it is neither Sunday nor Wednesday which she knew already before. Consequently her guess still cannot be better (or worse) than .5.

3. Jun 2, 2017

### Demystifier

Let me present an argument for 1/3, by analysing a similar problem from my actual real experience. Let us divide a day into a two equal halves, namely the range 24:00-12:00 and the range 12:00-24:00. When I awake during a sleep, at first I am not fully conscious, so at first I don't know what part of the day it is. Nevertheless, almost always I automatically assume that I am somewhere in the 24:00-12:00 range. And almost always I turn out to be right, despite the fact that, a priori, the two ranges should be equally likely. That's justified because I rarely go to sleep in the afternoon. (But sometimes I do go to sleep at afternoon hours, and in such cases, when I awake, at first I get confused when I see what time it is.)

4. Jun 2, 2017

### Demystifier

Are you a genuine halfer, or just a devil's advocate?

Anyway, perhaps the best way to see why 1/3 is the right answer is to go to the extreme. Instead of awaking the Beauty n=2 times, let us awake her n=1.000.000 times in the case of tails (and only k=1 times in the case of heads). So when she is awake, she can be almost certain that it was tails, so the probability for heads must be very small. And if n=1.000.000 is not convincing enough, consider the limit $n\rightarrow\infty$. If this is still not enough, vary also k and consider the limit $k\rightarrow 0$.

5. Jun 2, 2017

### Staff: Mentor

But this changes her a priori chances: she knows beforehand that heads is basically impossible. If we agree on the fact, that the two probabilities (before and after) don't differ, then the question is only what she can say on Sunday, even without the entire experiment. And these chances are even. I simply don't see how additional information should enter the system, like it does in the three doors riddle.

6. Jun 2, 2017

### PeroK

It depends how you define "credence". You could extend the problem thus:

If she guesses wrong, then another coin is tossed and if it's tails she is executed. Now, if she guesses heads every time, then she has a 50-50 chance of being wrong but only once. However, if she guesses tails every time she has a 50-50 chance of being wrong, but she will be wrong twice, so has an increased chance of execution.

I would compare it to a similar scenario: toss a coin and ask one or two different people at random. If the people know this rule then the fact that they have been picked favours heads. If they don't know this rule, they must go 50-50.

The princess is effectively two different people on account of the amnesia.

7. Jun 2, 2017

### PeroK

PS change the rule so that she gets woken every day for a year (heads) or only once a year (tails). I think that blows the 50-50 argument.

8. Jun 2, 2017

### Staff: Mentor

So if you have a different person every day of the year, it is still 50:50 for each single person assuming they cannot communicate.

9. Jun 2, 2017

### PeroK

Not if they know the rule about two or more people getting woken if it's heads. I see I got the role of heads and tails mixed up, but if I persevere with the way I wrote it.

If you toss a coin and if it's heads you ask everyone in Belgium, but if it's tails you pick one person at random and ask only them. If everyone knows this rule, even if they cannot communicate, they know that by virtue of being asked it is almost certainly heads.

If there were a cash prize for being right, it would be foolish to guess tails.

10. Jun 2, 2017

### PeroK

PS having read a bit more. One of the halfer arguments is that the princess "learns nothing new" about the coin when she is woken, so should stick with 1/2. Therefore, being woken involves learning nothing new.

Now, change the problem so that she is only woken if it is heads and not tails. She has still learned nothing new if she is woken, so sticks with 1/2, even though it's now 100% heads.

11. Jun 2, 2017

### TeethWhitener

I don't think this is obvious at all. The probability that she will be woken up is 1 (regardless of whether the coin is heads or tails). She has no way of knowing whether she has previously been woken up, so I don't see how having n=a zillion, k=1 changes anything at all. I think of it this way: let's say we have a two-day period. We flip a coin, and if it's heads, sleeping beauty is awake for the entire 2-day period. If it's tails, she's awake for a random subset of that period whose measure equals 1 day. At some point while sleeping beauty is awake, we receive a note in that period saying "Sleeping beauty is awake right now." At least as far as I'm interpreting the problem, we receive the note regardless of what the outcome of the coin toss is. I can't see how we've gained any knowledge about the situation by receiving the note, and yet to me, the problem is essentially equivalent to the initial problem.

12. Jun 2, 2017

### Staff: Mentor

All these counter arguments against the .5 fraction involve a change on the set up and the knowledge of it. Therefore it doesn't affect the original question with a .5 chance plus the argument, that there is no additional information available during the experiment. It's exactly this lack of information that makes the situation different from the standard three doors example for conditional probabilities.

13. Jun 2, 2017

### PeroK

The flaw in your argument is this: The question you imagine asking her at the start is:
If I toss a coin now, what is the probability it is heads? To which the answer is 1/2.

But, later, this is not precisely the question. The question now is: We have woken you up ...

If you asked her the same question at the beginning it would be:

If we toss a coin and wake you up twice if it's heads and once if it's tails, then at the time we wake you,what is the probability it is heads?

To which the answer is 2/3.

So, the answer to the precise question she is asked under the precise circumstances is 1/3 - 2/3. It is never 1/2 to begin with.

That's the fundamental flaw. The answer to the question is not 1/2 in the first place, so has no need to change. The information is all there at the outset about the precise circumstances under which the question will be asked.

The question is always asked under conditional circumstances. It is never asked under the unconditional circumstances of a single coin toss.

14. Jun 2, 2017

### PeroK

PS and, moreover, it's never 1/2 for the experimenters. It's always known to them by the time they wake her and ask the question.

When the question is asked the coin toss is known. But, the princess has the knowledge she always had that when she is wakened the coin is more likely to already have been a head than a tail.

The experimenters know this because they know that if they are waking her it is more likely to have been a head.

15. Jun 2, 2017

### Staff: Mentor

Let's take the Wikipedia setting: heads = Monday only ; tails = Monday and Tuesday
The chances to get awoken twice equals the chances to get awoken once. That doubles the chances for a Monday.
However, in case of Monday she still don't know anything about the coin flip. But she will be asked: "What is your credence now for the proposition that the coin landed heads?" So although the chances for Monday are biased, the chances for the coin flip are not.

16. Jun 2, 2017

### PeroK

PPS here's my solution:

At the outset, the experimenters and the princess have the same information about the experiment.

You ask the experimenters first: If you are waking the princess, what is the probability the coin is a head. This is clearly 1/3.

But, as the princess has the same information at the start, she must answer the same:

If you are being woken, what is the probability the coin is heads. She must answer 1/3.

When she is being woken, nothing has changed for her - no new info - so she sticks with 1/3.

It's an illusion that the probability is ever 1/2 and must somehow change to 1/3.

17. Jun 2, 2017

### Staff: Mentor

This is what I don't see. The chances are 50:50 on Tuesday and 50:50 on not Tuesday as rest. But I get the feeling this riddle is very similar to one I once read in a little book from Martin Gardner. A criminal has been sentenced to death within a week and the judge says, he won't know the day. His lawyer celebrated this as a victory, because it obviously can't be on Sunday, which would be the last day and in which case he knew on Saturday. But for the same reason it can't be Saturday for he would know on Friday and so on. Finally he was very surprised as it happened on Wednesday morning.

18. Jun 2, 2017

### PeroK

That's just elementary conditional probability theory, surely. No tricks. No embellishments.

The conditional probability of a head given the instance of a waking.

You can do it with a simple probability tree.

19. Jun 2, 2017

### Staff: Mentor

I know. That's why the day of the week is 2:1, but that doesn't affect the coin flip, which remains 1:1. And she isn't asked for the day, only for the coin.

20. Jun 2, 2017

### PeroK

If you take away all the confusing complications you are left with - for the experiment at least - a simple conditional probability. You can't look at it as a simple coin flip. It's the conditional probability of heads given some event.

Effectively you have a random event that occurs twice after a tail and once after a head. The conditional probability of a head, given the event is 1/3.

This is basic stuff. The problem has muddled this - with its talk of sleeping beauty - and obscured this.

The coin flip isn't affected as such by the event, but the probability the event resulted from a head is not necessarily 1/2.

For the experimenters, how is this problem different from any conditional probability?