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Comment author: Conrad 12 March 2008 07:48:22PM 0 points [-]

GBM:: ..That said, when I say a die has a 1/6 probability of landing on a 3, that means: Over a series of rolls in which no effort is made to systematically control the outcome (e.g. by always starting with 3 facing up before tossing the die), the die will land on a 3 about 1 in 6 times.::

--Well, no: it does mean that, but don't let's get tripped up that a measure of probability requires a series of trials. It has that same probability even for one roll. It's a consequence of the physics of the system, that there are 6 stable distinguishable end-states and explosively many intermediate states, transitioning amongst each other chaotically.


Comment author: Conrad 12 March 2008 07:36:26PM 0 points [-]

::Okay, so unpack "ungrounded" for me. You've used the phrases "probability" and "calculated or measured likelihood of heads coming up", but I'm not sure how you're defining them.::

Ungrounded: That was a good movie. Grounded: That movie made money for the investors. Alternatively: I enjoyed it and recommend it. -- is for most purposes grounded enough.

::I'm going to do two things. First, I'm going to Taboo "probability" and "likelihood" (for myself -- you too, if you want). Second, I'm going to ask you exactly which specific observable event it is we're talking about. (First toss? Twenty-third toss? Infinite collection of tosses?) I have a definite feeling that our disagreement is about word usage.::

You yourself said that we're dealing with one throw of a rigged coin, of unknown riggage. I don't think we have have a disagreement, exactly, except it looks to me like the discussion's moot.

But look: if I can back up a bit, the notion that we can be dealing with a rigged coin, know that it's rigged, and say that the --er, chances-- of getting a heads is "really" 50%, because we Just Don't Know, is useless. At that point you're using 50-50 because we have two possible known outcomes:

But in fact we deal with unknown probabilities *all the time*. Probabilities are by default unknown, until we measure them by repeated trial and a lot of scratch-work. What about when you're dealing with a medication that might kill someone, or not: in the absence of any information, do you say that's 50-50?


Comment author: Conrad 12 March 2008 06:31:18PM 0 points [-]

"Suppose our information about bias in favour of heads is equivalent to our information about bias in favour of tail. Our pdf for the long-run frequency will be symmetrical about 0.5 and its expectation (which is the probability in any single toss) must also be 0.5. It is quite possible for an expectation to take a value which has zero probability density."

What I said: if all you know is that it's a trick coin, you can lay even odds on heads.

"We can refuse to believe that the long-run frequency will converge to exactly 0.5 while simultaneously holding a probability of 0.5 for any specific single toss in isolation."

Again what I said: if the question is, "This is a trick coin: I've rigged it. I have written down here the probability that it'll come up heads. Do you accept that the number I've written down is .5?" -- You've got to say no. Since they've just told you it was rigged.

And if what they've written down is .50000000000001 and come back at you for it, then they stretched a point to say it was rigged.

So your problem is you haven't grounded the example in terms of what we're being asked to do.

Again, what difference does it make?


ps - Ofc, knowing, or even just suspecting, the coin is rigged, on the *second* throw you'd best bet on a repeat of the outcome of the *first*.


Comment author: Conrad 12 March 2008 05:00:44PM 0 points [-]

Maybe I'm stupid here... what difference does it make?

Sure, if we had a coin-flip-predicting robot with quick eyes it might be able to guess right/predict the outcome 90% of the time. And if we were precognitive we could clean up at Vegas.

In terms of non-hypothetical real decisions that confront people, what is the outcome of this line of reasoning? What do you suggest people do differently and in what context? Mark cards?

B/c currently, as far as I can see, you're saying, "The coin won't end up 'heads or tails' -- it'll end up heads, or it'll end up tails." True but uninformative.


ps - The thought experiment with the trick coin is ungrounded. If I'm being asked to lay even odds on a dollar bet that the coin is heads, then that's rational -- since the coin could be biased for heads, or tails (and the guy proposing the bet doesn't know the bias). If I'm being asked to accept or reject a number meant to correspond to the calculated or measured likelihood of heads coming up, and I trust the information about it being biased, then the only correct move is to reject the 0.5 probability. It has nothing to do with frequentist, Bayesian, or any other suchlike.