According to this page, only 6.4% of offenders in sex related homicides are female (lower than the 11.2% of all homicide offenders who are female). Of all homicides only 2.4% are female on female. I can’t see a simple way to derive the percentage of female offenders in sex related homicides with a female victim but it seems likely based on the other numbers to be lower than p(Female Offender|Female Victim) which I make to be 9.6%: p(FO|FV) = p(FV|FO)*p(FO)/p(FV) = 2.4 / (2.4 + 22.7) = 0.096.
So given no other information, if you know you have a female victim in a sex related homicide it would be reasonable to assume that it is at least 10 times more likely that the murderer is male. About 91% of female murder victims are killed by someone known to the victim so it is reasonable for the police to start with her close associates. Given no particular reason to favour Knox or Sollecito as the murderer you would put much higher odds on it being Sollecito purely based on his gender.
That was roughly the reasoning I used with my initial estimates. I hadn’t looked up any statistics at that point, I just knew that a female murder victim was significantly more likely to have been killed by a male than by a female, especially if there appeared to be a sexual motive to the murder. In light of the statistics I think if anything I should have put a wider gap on the estimates in the absence of other evidence implicating one over the other.
About 91% of female murder victims are killed by someone known to the victim so it is reasonable for the police to start with her close associates.
I don’t know anything about this specific case, but in general—we need to adjust this kind of statement for selection bias, and I would like to know how this is done.
When killers aren’t known to the victims, the police are less likely to find them both because they don’t look for them (as you said) and because there are too many people the victim didn’t know for the police to examine more than a small fraction.
Therefore I expect the just conviction rate in these crimes to be lower, and the false conviction rate to be higher (the police are always likely to accuse and help convict someone the victim knew, but in these cases we know such an accused is innocent).
However, your statistic (many murder victims knew their attacker) probably actually counts convictions. How should we revise this due to A) false convictions and B) a different-than-general rate of crimes solved (where conviction was achieved, or the primary suspect died or fled)? If the statistic already takes such considerations into account, how it this done?
According to this page, only 6.4% of offenders in sex related homicides are female (lower than the 11.2% of all homicide offenders who are female). Of all homicides only 2.4% are female on female. I can’t see a simple way to derive the percentage of female offenders in sex related homicides with a female victim but it seems likely based on the other numbers to be lower than p(Female Offender|Female Victim) which I make to be 9.6%: p(FO|FV) = p(FV|FO)*p(FO)/p(FV) = 2.4 / (2.4 + 22.7) = 0.096.
So given no other information, if you know you have a female victim in a sex related homicide it would be reasonable to assume that it is at least 10 times more likely that the murderer is male. About 91% of female murder victims are killed by someone known to the victim so it is reasonable for the police to start with her close associates. Given no particular reason to favour Knox or Sollecito as the murderer you would put much higher odds on it being Sollecito purely based on his gender.
That was roughly the reasoning I used with my initial estimates. I hadn’t looked up any statistics at that point, I just knew that a female murder victim was significantly more likely to have been killed by a male than by a female, especially if there appeared to be a sexual motive to the murder. In light of the statistics I think if anything I should have put a wider gap on the estimates in the absence of other evidence implicating one over the other.
I don’t know anything about this specific case, but in general—we need to adjust this kind of statement for selection bias, and I would like to know how this is done.
When killers aren’t known to the victims, the police are less likely to find them both because they don’t look for them (as you said) and because there are too many people the victim didn’t know for the police to examine more than a small fraction.
Therefore I expect the just conviction rate in these crimes to be lower, and the false conviction rate to be higher (the police are always likely to accuse and help convict someone the victim knew, but in these cases we know such an accused is innocent).
However, your statistic (many murder victims knew their attacker) probably actually counts convictions. How should we revise this due to A) false convictions and B) a different-than-general rate of crimes solved (where conviction was achieved, or the primary suspect died or fled)? If the statistic already takes such considerations into account, how it this done?