What Is the Probability of a Third Head After Two Heads with a Biased Coin?

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Discussion Overview

The discussion centers on the probability of obtaining a third head after observing two heads (HH) when flipping a biased coin, where the bias is determined by a random variable p drawn from a uniform distribution U[0,1]. The participants explore the implications of this setup and the expected value of a bet based on the outcome of the third flip.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the probability of heads on a single flip is defined by a random variable p, leading to a discussion about the implications of this definition.
  • Others argue that the results of the first two flips (HH) do not provide additional information about p, suggesting that the expected value of a bet based on the third flip is simply p.
  • A later reply questions the validity of the bet's worth, stating that the probability of selecting a rational number from the uniform distribution is zero, complicating the valuation of the bet.
  • Another participant suggests rounding p to the nearest 1/100 to facilitate the calculation, indicating a practical approach to the problem.
  • One participant calculates the probability of HHH given HH, proposing that it involves the expectation value of p^3 and provides integrals to support their reasoning, arriving at a probability of 3/4 for the third head.
  • Another participant expresses agreement with the calculated probability, indicating a level of acceptance of that specific interpretation.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the initial conditions and the nature of the probability involved. There is no consensus on the valuation of the bet or the interpretation of the results, indicating ongoing debate and exploration of the topic.

Contextual Notes

The discussion includes assumptions about the nature of p and its distribution, as well as the implications of rational versus irrational numbers in the context of the bet's worth. These aspects remain unresolved and are subject to interpretation.

davidmoore63@y
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A random number p such that 0<p<1 is selected at random from a uniform distribution U[0,1]. A biased coin is then constructed such that the probability of heads on a single flip is p (thus 1-p for a tails).

This coin is flipped twice and the result is HH. If the coin is flipped a third time, what is the probability of a third head? More precisely, what is the fair value of a lottery ticket that pays one dollar if the third flip is a head, and zero otherwise? What would you pay for it/ sell it for?
 
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Is this a trick question?

"A biased coin is then constructed such that the probability of heads on a single flip is p..."
 
not at all! I think it's well defined isn't it?
 
Then the two H results don't give you any additional information. You know the probability is p, and you know the expected value of a $1 bet is p x $1.
 
The question is asking for the probability of a third head PRIOR to finding out what p is.
 
the bet is worth $0 as the probability of drawing a rational number from the uniform distribution is zero and you can't pay someone and irrational number's worth of currency ;)
 
Ok for you we round p to the nearest 1/100. You still have to do the question now!
 
We want to know the probability of HHH given HH. It's Prob(HHH) / Prob(HH). The probability of HHH is the expectation value of p^3 where p is uniformly distributed on the interval [0, 1]. So it's int_0^1 p^3 dp = 1/4. Similarly Prob(HH) = int_0^1 p^2 dp = 1/3. So the answer is 3/4.
 
Looks right to me
 

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