4 ms·

This doesn't make sense. For two reasons. (a) It isn't zero-sum. Guede was convicted -- he is guilty, he did it. What we are determining is whether he acted

by ezy 17y ago

This doesn't make sense. For two reasons. (a) It isn't zero-sum. Guede was convicted -- he is guilty, he did it. What we are determining is whether he acted alone. (b) The priors are crap because they are weak, not because they are agreed upon. This is just a model, and it feels to me that the author is overfitting. The prior you mentioned as an example is so far away from the case at hand to be virtually useless.

frisco 17y ago
So, you're right in that Guede being guilty doesn't imply that Knox isn't. But, the evidence against him affects how the evidence against Knox has to be handled: for example, the lack of physical evidence against Knox becomes a big deal when Guede leaves tons of it. Re: priors being far away: a prior is the probability of an event occurring before any evidence has been observed. Priors are always necessarily that far removed; those are the correct priors.
- zaphar 17y ago
  How exactly does the evidence against Guede affect the regarding of evidence against Knox? Assuming that everyone involved (if it was multiple parties) were equally involved equally competent and hiding their involvement and left equal evidence of their involvement is just that an "assumption". It's not necessarily true and is in fact highly unlikely to be true. In the article actual physical evidence is dismissed because it's "small in quantity". The amount of blood is far less important than whose blood it is. You only need a little dna to prove contact. And you only need a little blood to prove prescence at the scene of an injury. It sounds to me like the jury was doing it's job and the article's author was just looking for a point to make.
  - demallien 17y ago
    But it isn't just that it's "small in quantity" that means that there is a problem. Amanda was a housemate, it would be incredible if her DNA couldn't be found in her housemate's room. The simplest explanation for Amanda's DNA being in the room is that she had previously spent some time there talking to Meredith. To put this another way, have a think about where you would expect your own DNA to show up. At home, at your partner's appartment, at the office, at the cafe around the corner from the office, in your car, in your partner's car, at your best friend's home, your best friend's car. If a murder ever took place in one of these locations, how would you feel about the presence of your DNA being used to implicate you in the murder?
- ezy 17y ago
  Fair enough, I could see relative evidence being used as evidence. :-) I think that negates my first point, you are correct. But I strongly disagree on your understanding of the impact of priors. There are several ways to look at it, all equivalent. The easiest is the single prior: Did she do it? None of us are psychic, so that isn't helpful, 'cause we cant estimate it at all. We have to break that down into some of its components. In fact, your argument (which I totally agree with, BTW) is based on a very specific prior: that in cases with this kind of relative imbalance of evidence, there is a high prob that one party is guilty and the other isn't (or some such, I may not be stating it as specifically as I should be). All the evidence presented has it's own implied priors, and the right combination of evidence has it's own prior, etc. These all need to be combined with weights based on other priors. Since noone can compute all of these priors in the space of one hour, that makes it all a judgment call, not a rational decision. And this is where I think the post's pontificating gets it terribly wrong. There's nothing rational at all about the process, other than you're attempting to blindly apply the Bayesian formula.
  - frisco 17y ago
    Ok. I agree with you that the post was confused and kind of incoherent at times. It was somewhere deep in it I ended up seeing what the author was talking about (taking his statements about physical evidence on faith for the sake of discussion). I'm definitely not saying that Ms Knox is innocent -- I don't know anything about the facts of that case, so we're only talking about the mathematical methodology here. I do think your definition of a prior is a bit off though... like, the choices around priors wouldn't be whether to use "did she do it" vs the imbalances of evidence or such. We're trying to estimate the likelihood of "did she do it," so the question is what do we use to do that? The prior probability of "she did it" has to be the probability that some random member of the 20/female/american/student general public will commit the murder, because we don't have anything more than that (as you said). Then, we update our estimate by looking at the available evidence (such as, "was found standing by door holding bleach mop," which would the viewed in terms of the likelihood that that would be observed and the statement 'she did not do it' being true) to come to our posterior probability, which is the estimate informed by all of the available evidence. As a side note, it's great when you can have a civil discussion on the internet. Maybe there's hope for HN after all.

by ezy 17y ago

frisco 17y ago

So, you're right in that Guede being guilty doesn't imply that Knox isn't. But, the evidence against him affects how the evidence against Knox has to be handled: for example, the lack of physical evidence against Knox becomes a big deal when Guede leaves tons of it. Re: priors being far away: a prior is the probability of an event occurring before any evidence has been observed. Priors are always necessarily that far removed; those are the correct priors.

zaphar 17y ago
How exactly does the evidence against Guede affect the regarding of evidence against Knox? Assuming that everyone involved (if it was multiple parties) were equally involved equally competent and hiding their involvement and left equal evidence of their involvement is just that an "assumption". It's not necessarily true and is in fact highly unlikely to be true. In the article actual physical evidence is dismissed because it's "small in quantity". The amount of blood is far less important than whose blood it is. You only need a little dna to prove contact. And you only need a little blood to prove prescence at the scene of an injury. It sounds to me like the jury was doing it's job and the article's author was just looking for a point to make.
- demallien 17y ago
  But it isn't just that it's "small in quantity" that means that there is a problem. Amanda was a housemate, it would be incredible if her DNA couldn't be found in her housemate's room. The simplest explanation for Amanda's DNA being in the room is that she had previously spent some time there talking to Meredith. To put this another way, have a think about where you would expect your own DNA to show up. At home, at your partner's appartment, at the office, at the cafe around the corner from the office, in your car, in your partner's car, at your best friend's home, your best friend's car. If a murder ever took place in one of these locations, how would you feel about the presence of your DNA being used to implicate you in the murder?
ezy 17y ago
Fair enough, I could see relative evidence being used as evidence. :-) I think that negates my first point, you are correct. But I strongly disagree on your understanding of the impact of priors. There are several ways to look at it, all equivalent. The easiest is the single prior: Did she do it? None of us are psychic, so that isn't helpful, 'cause we cant estimate it at all. We have to break that down into some of its components. In fact, your argument (which I totally agree with, BTW) is based on a very specific prior: that in cases with this kind of relative imbalance of evidence, there is a high prob that one party is guilty and the other isn't (or some such, I may not be stating it as specifically as I should be). All the evidence presented has it's own implied priors, and the right combination of evidence has it's own prior, etc. These all need to be combined with weights based on other priors. Since noone can compute all of these priors in the space of one hour, that makes it all a judgment call, not a rational decision. And this is where I think the post's pontificating gets it terribly wrong. There's nothing rational at all about the process, other than you're attempting to blindly apply the Bayesian formula.
- frisco 17y ago
  Ok. I agree with you that the post was confused and kind of incoherent at times. It was somewhere deep in it I ended up seeing what the author was talking about (taking his statements about physical evidence on faith for the sake of discussion). I'm definitely not saying that Ms Knox is innocent -- I don't know anything about the facts of that case, so we're only talking about the mathematical methodology here. I do think your definition of a prior is a bit off though... like, the choices around priors wouldn't be whether to use "did she do it" vs the imbalances of evidence or such. We're trying to estimate the likelihood of "did she do it," so the question is what do we use to do that? The prior probability of "she did it" has to be the probability that some random member of the 20/female/american/student general public will commit the murder, because we don't have anything more than that (as you said). Then, we update our estimate by looking at the available evidence (such as, "was found standing by door holding bleach mop," which would the viewed in terms of the likelihood that that would be observed and the statement 'she did not do it' being true) to come to our posterior probability, which is the estimate informed by all of the available evidence. As a side note, it's great when you can have a civil discussion on the internet. Maybe there's hope for HN after all.