That’s setting the bar pretty high. What about witnesses who claim to have seen two men running from the scene, or heard multiple voices at the time of the murder?
pete22
Karma: 103
You’re right, this is the part I have trouble understanding. Partly because I have trouble separating the facts of this particular case from the theoretical point you’re making. So if you’ll humor me for a second: let’s say you’re the chief of police somewhere, and one of your detectives comes back from a murder scene and tells you “we’ve arrested a prime suspect and we’ve got very strong evidence against him, including an ironclad DNA match that ties him to the murder. but we don’t think he acted alone, because of X, so we want to keep looking for another suspect.”
It sounds like for almost any X, your response would be “Don’t waste your time. Case closed. Your hypothesis that there was another killer is unnecessary to explain the victim’s death.” Is that correct? What kind of X would suffice for you to let your detectives keep investigating?
By far the most important evidence in a murder investigation will therefore be the evidence that is the closest to the crime itself—evidence on and around the victim, as well as details stored in the brains of people who were present during the act.
the evidence against Guédé is such that the hypothesis of her guilt is superfluous—not needed—in explaining the death of Meredith Kercher
I think you’re begging the question here. Those who are convinced that K&S are guilty seem to believe that the evidence from the crime scene itself suggests that Guede could not have been the only participant – i.e. his involvement does NOT completely explain the death of Meredith Kercher. They seem to believe this for two reasons:
The various evidence at the crime scene itself of a clean up and/or staged break-in, things like the bra being cut off or the body being moved – hard to attribute to Guede because a. he wouldn’t have had time (is this true?), and b. little effort was taken to remove evidence against Guede himself, even very obvious things like flushing the toilet.
I haven’t seen any commenters mention it here, but one of the anti-Amanda sites, in quoting the Micheli report, seemed to imply that Kercher’s injuries were inconsistent with a single attacker.
Your post appears to take for granted that these are not credible arguments, but they seem like a very significant part of the prosecution’s case – and the part that I found hardest to assess without English versions of the Micheli report and other source documents. Can you explain how you reached this level of confidence?
FWIW, I’m not an Amanda-hater …my prior probability of her guilt was 30%, and I’m ready to revise it down. I’m just not sure I fully understand your argument on this point.
I just went to Wikipedia and found a more articulate version of what I’m trying to say:
Gardner-Medwin argues that the criterion on which a verdict in a criminal trial should be based is not the probability of guilt, but rather the probability of the evidence, given that the defendant is innocent (akin to a frequentist p-value). He argues that if the posterior probability of guilt is to be computed by Bayes’ theorem, the prior probability of guilt must be known. This will depend on the incidence of the crime, which is an unusual piece of evidence to consider in a criminal trial. Consider the following three propositions:
A: The known facts and testimony could have arisen if the defendant is guilty,
B: The known facts and testimony could have arisen if the defendant is innocent,
C: The defendant is guilty.
Gardner-Medwin argues that the jury should believe both A and not-B in order to convict. A and not-B implies the truth of C, but the reverse is not true. It is possible that B and C are both true, but in this case he argues that a jury should acquit, even though they know that they will be letting some guilty people go free. See also Lindley’s paradox.
I am not really a stats person and I’m not prepared to defend Garder-Medwin’s model as being correct—but right or wrong, it’s a better description than Bayesian inference of most people’s intuitive concept of the task of a juror.
In other words, when I imagine myself as a juror I’m automatically more concerned about a false positive (convicting an innocent person), and I will intuitively try to answer the question “has the prosecution proved its case” rather than “is this person guilty.”
If asked to answer the second question and quantify my odds of guilt, I’m likely to understate them, precisely because I can’t separate that estimate from the real-world effect of a guilty verdict.
Or in your terms, the “question of what probability corresponds to ‘beyond reasonable doubt’ [or whatever the equivalent standard in Italy]” can’t be completely excluded from the question when we imagine ourselves as jurors, only made implicit.
This reminds me slightly of Eliezer’s “true Prisoner’s Dilemma” article, which I really liked. Just as you can’t posit that someone is my confederate (in his case) and then ask me to consider them in a purely selfish, impartial way—you can’t tell me I’m a juror and then ask me to make a purely impartial assessment. I’m describing a much weaker effect than he was, and maybe it’s more socially conditioned than inherent to human nature, but I think the general concept is the same.
So …better to say “forget the fact that there’s even a trial going on, just imagine that tomorrow the absolute truth will be revealed and you have to bet on it now.”
I have a different objection to the premise: the presumption of innocence in modern legal systems means that the job of the jury (and by extension the legal teams) is not just to arrive at a probability of guilt but at a certain level of confidence around that probability.
I realize that these can technically be made equivalent by endless “priors”—i.e. a juror walks into the courtroom with a certain set of beliefs, ie probability .8 that someone on trial is guilty, .01 that a sex crime would have been committed by a female rather than a male, etc etc etc, and mentally revises them according to every input—but in any normal intuitive sense the job of a juror is not to answer the exact questions you’ve presented, but to judge the strength of the prosecution’s case from a skeptical viewpoint.
So when you conflate the process/outcome of the trial with an independent, rational probability estimate, I think you make it hard for anyone to give you an impartial answer. And this starts right in the title with “you be the juror”—but it extends all the way to minor points like the use of the term “guilty” which has both legal and colloquial meanings.
Come to think of it, why would a further (unknown) DNA sample even be enough? You’ve got an explanation for the crime. Surely there are far more likely priors than a double murder that would explain a second DNA sample, right? “Similarly incriminating” might not really be possible; much of what made the first sample so incriminating is that it matched a guy who you already had reason to suspect. Or did you mean that an unknown DNA sample would not suffice, only one that matched another suspect?