S
Sniper8052(L96A1)
Guest
Tony Raven wrote:
> Sniper8052(L96A1) wrote on 26/11/2006 11:44 +0100:
>
>>
>> If I cannot disprove the alibi, 'he was at home in bed', I have no
>> means of improving my case and the offender has to be found not guilty.
>>
>> Given the above I still prefer the use as in Theorem 1. The offender
>> is in there somewhere but with legal safeguards. I still think that
>> DNA used in this method would offer a significant safeguard against
>> many offences.
>>
>
> Wrong. You don't even know if the offender is in your 650,000 your way
> and at least one of them is bound to be unable to pass your other
> subjective tests even though they are innocent e.g. they have no alibi
> for where they were at the time. The beauty of the Bayes 2 scenario is
> that you don't need a national database. Once you've identified someone
> by the other characteristics you can take their DNA and test it against
> the evidence.
>
> Lets take another slightly more controversial proposal. Lets have a
> database of everyone by race. Then when a person commits a crime and
> the evidence from the CCTV is that he is Chinese, you can search your
> database for all the people that are Chinese. Then by your rules, all
> you have to do is find a Chinese person who can't show an alibi and
> looks like the person on the CCTV. Do you think that will have caught
> you your criminal?
>
> The correct way to do it is use your normal investigative skills to
> identify a list of suspects from whom you can eliminate those that don't
> look like the person on the CCTV. Then you have a small number of
> people left for whom you have to show by other means which one it is.
> That way your false positive rate is very much lower.
>
All,
I have just skimmed through some of your replies, for which I am
extremely grateful; not least because by increasing my understanding of
the fallibility of the DNA system you are helping me to do my job more
fairly and be more aware of the possibility of error.
I took the providence of the DNA sample as being proved throughout this
discussion but it is self evident that it's providence must be shown in
the chain of evidence for it to have any value.
I had however made an assumption that the suspect would be included
within the 1% error. This was a mistake that I now see - target
fixation - and the problem that this creates is one which seems
insurmountable in Theorem 1. Does this calls into question the 'cold
case' use of DNA results or would the effect reduce over time - decades
rather than months/years?
It is unfortunate that a test which is right 99% of the time cannot be
relied upon when used at a population level, based upon these
statistical models, for surely this would be an ideal deterrent to crime
and aid to its detection: my motive in our discussion has been the
discouragement of violent and sexual crime through such use hence my
determination for its use at national level through an ID system.
I wonder, are we sure that the 1% figure represents false positives in
this discussion. I ask as others have talked of false negatives and if
it were the case that a false negative figure of 1% is returned then the
debate has been erroneous.
What is the figure for false negatives anyway if the 1% is correct as
false positives?
Sniper8052
> Sniper8052(L96A1) wrote on 26/11/2006 11:44 +0100:
>
>>
>> If I cannot disprove the alibi, 'he was at home in bed', I have no
>> means of improving my case and the offender has to be found not guilty.
>>
>> Given the above I still prefer the use as in Theorem 1. The offender
>> is in there somewhere but with legal safeguards. I still think that
>> DNA used in this method would offer a significant safeguard against
>> many offences.
>>
>
> Wrong. You don't even know if the offender is in your 650,000 your way
> and at least one of them is bound to be unable to pass your other
> subjective tests even though they are innocent e.g. they have no alibi
> for where they were at the time. The beauty of the Bayes 2 scenario is
> that you don't need a national database. Once you've identified someone
> by the other characteristics you can take their DNA and test it against
> the evidence.
>
> Lets take another slightly more controversial proposal. Lets have a
> database of everyone by race. Then when a person commits a crime and
> the evidence from the CCTV is that he is Chinese, you can search your
> database for all the people that are Chinese. Then by your rules, all
> you have to do is find a Chinese person who can't show an alibi and
> looks like the person on the CCTV. Do you think that will have caught
> you your criminal?
>
> The correct way to do it is use your normal investigative skills to
> identify a list of suspects from whom you can eliminate those that don't
> look like the person on the CCTV. Then you have a small number of
> people left for whom you have to show by other means which one it is.
> That way your false positive rate is very much lower.
>
All,
I have just skimmed through some of your replies, for which I am
extremely grateful; not least because by increasing my understanding of
the fallibility of the DNA system you are helping me to do my job more
fairly and be more aware of the possibility of error.
I took the providence of the DNA sample as being proved throughout this
discussion but it is self evident that it's providence must be shown in
the chain of evidence for it to have any value.
I had however made an assumption that the suspect would be included
within the 1% error. This was a mistake that I now see - target
fixation - and the problem that this creates is one which seems
insurmountable in Theorem 1. Does this calls into question the 'cold
case' use of DNA results or would the effect reduce over time - decades
rather than months/years?
It is unfortunate that a test which is right 99% of the time cannot be
relied upon when used at a population level, based upon these
statistical models, for surely this would be an ideal deterrent to crime
and aid to its detection: my motive in our discussion has been the
discouragement of violent and sexual crime through such use hence my
determination for its use at national level through an ID system.
I wonder, are we sure that the 1% figure represents false positives in
this discussion. I ask as others have talked of false negatives and if
it were the case that a false negative figure of 1% is returned then the
debate has been erroneous.
What is the figure for false negatives anyway if the 1% is correct as
false positives?
Sniper8052
