Epictetus comments on [LINK] Amanda Knox exonerated - Less Wrong

9 Post author: fortyeridania 28 March 2015 06:15AM

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Comment author: gwern 28 March 2015 07:43:19PM *  8 points [-]

That's still base-rate neglect as you are picking and choosing what you want to look at and not conditioning on one of the more relevant variables.

What fraction of the pretty girls who lived with the victims turned out to be murderers? By looking at the genderless conditional probability ('somebody'), you're implying that women like Knox might have male-like murder levels, which is obviously wrong. And to the extent that pretty girls do not have differing patterns of murdering roommates from other women, you're making the exact same mistake, even (it doesn't matter whether you update on pretty girl then roommate or roommate then pretty girl).

Update on both living with the victim and being female and the small probability is bigger but... still small, since the still relatively low probability of a roommate murdering is penalized substantially by being female (female murder rates are like 1/10th male and that's the raw rates, not adjusted for age or SES or race etc). As the top comment says, "Once we take into account that AK and MK aren't in a relationship, AK is female, and there is very strong evidence that someone else committed the murder then I'd agree that the probability drops".

Comment author: Epictetus 29 March 2015 07:02:04AM 5 points [-]

Update on both living with the victim and being female and the small probability is bigger but... still small, since the still relatively low probability of a roommate murdering is penalized substantially by being female

Low compared to what? For someone murdered at home in the dead of night, the dominant probabilities are that either the murderer was invited in or lived there. Roommates merit investigation. If the evidence clears spouses/lovers and close family, then the probability of it being a roommate goes up considerably. Being female is not going to lower the probability enough to eschew a thorough investigation.

What saves Amanda Knox in this case isn't being female, but rather evidence that someone else committed the crime, as well as the lack of physical evidence of her involvement or any paper trail pointing to a conspiracy.

Comment author: gwern 30 March 2015 12:09:53AM *  1 point [-]

For someone murdered at home in the dead of night, the dominant probabilities are that either the murderer was invited in or lived there. Roommates merit investigation. If the evidence clears spouses/lovers and close family, then the probability of it being a roommate goes up considerably. Being female is not going to lower the probability enough to eschew a thorough investigation.

You're not disagreeing, but you're failing to consider the numbers here. If, say, a quarter of people are murdered by their roommates, and males are 10x more likely to be killers than females, what's the odds of a female roommate doing it?

Being female is not going to lower the probability enough to eschew a thorough investigation.

A probability like 2.5% is worth following up on if police have no better leads to focus on, but they visibly focused on it way more, and in fact people focused on it way more; consider how many expressed probabilities were higher than that in the LW survey. And consider the implicit probabilities in the faction of the public and the Kirchers baying for Knox's blood.

All consistent with base-rate neglect (of being female).

Comment author: private_messaging 30 March 2015 10:16:34PM *  -1 points [-]

Okay, let's go with your number... let's suppose hypothetically that you aren't beating or otherwise unduly coercing cute girls into saying what you want, and you started with the probability of 2.5%. Then your suspect tells you they were at the house covering their ears not to hear the screams as their big black boss murdered the victim. Now what happens to 2.5%? After you clear the big black boss, what happens?

I don't think you can claim base rate neglect without also claiming police brutality, coercion, and leading the witness (which would be a much bigger problem)

Comment author: Epictetus 30 March 2015 04:07:44AM 2 points [-]

If, say, a quarter of people are murdered by their roommates, and males are 10x more likely to be killers than females, what's the odds of a female roommate doing it?

Depends on whether murder by roommate and murder by female are independent. An average taken over the all homicides includes gang violence, robberies, bar fights, etc. Some kinds of murders are overwhelmingly perpetrated by males, while others are more balanced (for example, males are only 50% more likely to kill their children than females). Once we narrow down the circumstances of the murder, all kinds of dependencies and conditionals start popping up and the base rate becomes less relevant.

A probability like 2.5% is worth following up on if police have no better leads to focus on, but they visibly focused on it way more, and in fact people focused on it way more; consider how many expressed probabilities were higher than that in the LW survey.

Agreed. Police try to shoehorn in their theory and the press isn't going to let the truth get in the way of a good story.

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