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This is a question about an alternative solution to the problem stated in Probability and exit polls

The problem is:

Consider an election with two candidates, Candidate A and Candidate B. Every voter is invited to participate in an exit poll, where they are asked whom they voted for; some accept and some refuse. For a randomly selected voter, let A be the event that they voted for A, and W the event that they are willing to participate in the exit poll. Suppose that P(W∣A)=0,7 but P(W∣AC)=0,3. In the exit poll, 60% of the respondents say they voted for A (assuming they are all honest), suggesting a comfortable victory for A. Find P(A).

The answer there uses Law of Total probability to arrive at an answer of 9/23. However, I'm confused why when I use the odds form of Bayse' rule, I get a completely different answer.

This is what I do:

$P({W}\mid{A})=0.7$ (probability of participating in exit poll given voting for A)

$P({W}\mid{A^c})=0.3$ (probability of participating in exit poll given voting for B)

$\frac{P({A}\mid{W})}{P({A^c}\mid{W})}=0.6$ (portion of respondents that say they voted for A in the exit poll)

I'm looking for $P(A)$. So, applying the odds form of Bayse' rule:

$\frac{P({A}\mid{W})}{P({A^c}\mid{W})}=\frac{P({W}\mid{A})}{P({W}\mid{A^c})} * \frac{P(A)}{P(A^c)}$

$\frac{6}{10} = \frac{7}{3} * \frac{P(A)}{1-P(A)}$

$\frac{9}{35} = \frac{P(A)}{1-P(A)}$

$P(A)\approx0.2$

That's far off from $\frac{9}{23}$. I'm wondering where I'm going wrong?

matto
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1 Answers1

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$\frac{P({A}\mid{W})}{P({A^c}\mid{W})}=\frac{P({W}\mid{A})}{P({W}\mid{A^c})} * \frac{P(A)}{P(A^c)}$

Here $P({A}\mid{W}) = 0.6$ and $P({A^c}\mid{W}) = 0.4$

so equation should be $\frac{0.6}{0.4} = \frac{7}{3} \frac{x}{1-x}$ where $x = P(A)$. This gives $x = \frac{9}{23}$.

sku
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  • D'oh! So "In the exit poll, 60% of the respondents say they voted for A" does not mean "a randomly chosen respondent is 0.6 times more likely to have voted for A" and instead it means "a randomly chosen respondent has a 60% chance of having voted for A". This makes perfect sense now thank you! – matto Nov 22 '23 at 12:48