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And now even DNA is being called into question. http://mobile.nytimes.com/2013/09/17/science/dna-double-take.html http://mobile.nytimes.com/2013/09/17/science/

And now even DNA is being called into question. http://mobile.nytimes.com/2013/09/17/science/dna-double-take.html http://mobile.nytimes.com/2013/09/17/science/dna-double-take...

pasbesoin 13y ago
In addition to the chimeric qualities cited in the NYT article (I skimmed), IIRC some DNA sampling has in the past used and may still use a fairly limited profile of markers. The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. Never read into it in detail, but I was left with the impression that "unique identifier" can be an over-statement/qualification also from this perspective.
- AnthonyMouse 13y ago
  >The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. In addition to that, there are problems with bias and statistical independence. A given marker is unlikely to be present in exactly 50% of the population, and to the extent that it isn't it can reduce the probability by that amount that a test match is a true match. Meanwhile the suspect pool for a given crime is likely to encompass several (perhaps many) members of the same extended family, who for the obvious reason are significantly more likely than random members of the world population to have the tested markers match one another. Even within a city you will generally see concentrations of specific ethnicities who may have a higher statistical incidence of specific genetic markers than other populations, which can screw up the numbers by an amount that historically hasn't even knowable because we don't have good numbers on the statistical incidence of specific markers within geographical populations. The place where this is most pernicious is when they get a sample from a crime scene and run it against some "DNA database" to find a hit. Then everybody is talking about the probability that X suspect would match the DNA at the crime scene rather than the probability that someone in the database would match even if the actual perpetrator wasn't in the database.
  - coldtea 13y ago
    >The place where this is most pernicious is when they get a sample from a crime scene and run it against some "DNA database" to find a hit. Then everybody is talking about the probability that X suspect would match the DNA at the crime scene rather than the probability that someone in the database would match even if the actual perpetrator wasn't in the database. Or the probability that, match or not, the DNA from the crime scene belongs to the criminal. And not, say, someone the victim came in contact with earlier, someone that happened to be in the crime scene before the crime took place, or even some third guy the actual criminal took a DNA from in order to frame him.
- brazzy 13y ago
  > The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. I thought that was pretty much always the case. Which is why DNA evidence is never used alone - you don't take DNA traces found at a scene of crime, run it against a huge database, find a match, close the case and try and sentence the matching person. Instead you either investigate whether that matching person had means and motive and no alibi, or (more often) you check the DNA only against people you already suspect for whatever reasons. Both variants reduce the likelihood of false positives by quite a few orders of magnitude.
- tingletech 13y ago
  I thought they used restriction digests with gel electrophoresis, I'm pretty sure full genomic sequencing would be too expensive.
  - pasbesoin 13y ago
    I don't really know, but this does sound familiar and is part of what I was speaking to. What type and generation of technology was used? How much was the solicitor of the test willing to pay for it (influencing the choice made within the current range of available technologies/capabilities), as well as how many sequence/data points were targeted. As one example, combine a fairly limited set of targets with gel chromatography, and varying quality/accuracy of analysis/analysts of same... And you have a lot less "uniqueness" than things like the common, public term "DNA fingerprint" imply. Yes, it may be a useful tool in combination with proper understanding of its limitations. However, we have (in the U.S., for example) and adversarial judicial process and prosecutors have been shown to often not place such understanding even in context let alone as a primary concern. If the defence is lacking, including simply financially to engage its own "expert witnesses"... misbegotten interpretations can and do rule the day.

And now even DNA is being called into question. http://mobile.nytimes.com/2013/09/17/science/dna-double-take.html http://mobile.nytimes.com/2013/09/17/science/dna-double-take...

pasbesoin 13y ago

In addition to the chimeric qualities cited in the NYT article (I skimmed), IIRC some DNA sampling has in the past used and may still use a fairly limited profile of markers. The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. Never read into it in detail, but I was left with the impression that "unique identifier" can be an over-statement/qualification also from this perspective.

AnthonyMouse 13y ago
>The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. In addition to that, there are problems with bias and statistical independence. A given marker is unlikely to be present in exactly 50% of the population, and to the extent that it isn't it can reduce the probability by that amount that a test match is a true match. Meanwhile the suspect pool for a given crime is likely to encompass several (perhaps many) members of the same extended family, who for the obvious reason are significantly more likely than random members of the world population to have the tested markers match one another. Even within a city you will generally see concentrations of specific ethnicities who may have a higher statistical incidence of specific genetic markers than other populations, which can screw up the numbers by an amount that historically hasn't even knowable because we don't have good numbers on the statistical incidence of specific markers within geographical populations. The place where this is most pernicious is when they get a sample from a crime scene and run it against some "DNA database" to find a hit. Then everybody is talking about the probability that X suspect would match the DNA at the crime scene rather than the probability that someone in the database would match even if the actual perpetrator wasn't in the database.
- coldtea 13y ago
  >The place where this is most pernicious is when they get a sample from a crime scene and run it against some "DNA database" to find a hit. Then everybody is talking about the probability that X suspect would match the DNA at the crime scene rather than the probability that someone in the database would match even if the actual perpetrator wasn't in the database. Or the probability that, match or not, the DNA from the crime scene belongs to the criminal. And not, say, someone the victim came in contact with earlier, someone that happened to be in the crime scene before the crime took place, or even some third guy the actual criminal took a DNA from in order to frame him.
brazzy 13y ago
> The statistically likelihood of matches between distinct parties is in some cases well under the population of the world. I thought that was pretty much always the case. Which is why DNA evidence is never used alone - you don't take DNA traces found at a scene of crime, run it against a huge database, find a match, close the case and try and sentence the matching person. Instead you either investigate whether that matching person had means and motive and no alibi, or (more often) you check the DNA only against people you already suspect for whatever reasons. Both variants reduce the likelihood of false positives by quite a few orders of magnitude.
tingletech 13y ago
I thought they used restriction digests with gel electrophoresis, I'm pretty sure full genomic sequencing would be too expensive.
- pasbesoin 13y ago
  I don't really know, but this does sound familiar and is part of what I was speaking to. What type and generation of technology was used? How much was the solicitor of the test willing to pay for it (influencing the choice made within the current range of available technologies/capabilities), as well as how many sequence/data points were targeted. As one example, combine a fairly limited set of targets with gel chromatography, and varying quality/accuracy of analysis/analysts of same... And you have a lot less "uniqueness" than things like the common, public term "DNA fingerprint" imply. Yes, it may be a useful tool in combination with proper understanding of its limitations. However, we have (in the U.S., for example) and adversarial judicial process and prosecutors have been shown to often not place such understanding even in context let alone as a primary concern. If the defence is lacking, including simply financially to engage its own "expert witnesses"... misbegotten interpretations can and do rule the day.