Comments on: The METDI Defense

By: mirriam

mirriam — Sat, 12 Jun 2010 17:43:01 +0000

I wonder what comes after DNA.

By: Mark Draughn

Mark Draughn — Sat, 12 Jun 2010 00:32:49 +0000

I don’t know anything about the Myers case, but I think it depends on how the defendant came to the attention of of the police, and what else links him to the crime. If the initial suspect pool consists of the entire population of the United States, and the defendant just popped out of a CODIS hit, then the fact that there are about 190 other people who also have matching DNA is real important.

Even if the defendant lives in the same area as the victim, it’s not too meaningful. With 190 people scattered all over the country that match the DNA, one of them is bound to live within easy driving distance.

On the other hand, if the defendant works with the victim, and he kept asking her out, and she kept saying no, and witnesses heard him threaten her, and his car appears on a traffic camera three blocks from her house around the time of the murder, and he has a registered gun of the same caliber as the murder weapon, and his alibi doesn’t hold up, and then you get the 1 in 1.6 million DNA match…he’s probably the guy.

By: Jonathan C. Hansen

Jonathan C. Hansen — Fri, 11 Jun 2010 22:19:35 +0000

You know, I wanted to comment on this topic last night on Gamso’s blawg, but was too tired. Seeing this picked up by Windypundit reminded me. There is another aspect to these probability inferences that is often overlooked.
There is an excellent video regarding this topic on YouTube from the TED talks, that shows how totally unable people in general are at reasoning with statistics (http://www.youtube.com/watch?v=kLmzxmRcUTo).
For instance, in the Stephen Myers case, the probability of 1 in 1.6 million may well have been presented as there is only a one in 1.6 million chance that he didn’t do it. But that is not correct, as it is not the whole story. As pointed out in the video, one has to take into account another piece of information; in this case the “prevalence” of other people that have the same combination of DNA segments.
A one in 1.6 million chance, multiplied by, say 300 million as the number of people in the US, is 1 in about 187 that he DID do it. That is, given the large number of people in the US, there will be 187 or so others that have the same combination of DNA segments that were tested as does Stephen Meyers.
The video is very good, well worth watching, and certainly opened my eyes. I hope no one is offended if I post this comment on both blogs…