The LA times is flabbergasted that amongst large numbers of entries in a database:
- There are similar entries
- People who have been banking on people’s bad understanding of the statistics of large numbers are reluctant to have them re-educated about the reality of the situation.
The FBI DNA database has some close matches (strangers matching at 9 points of the DNA profile). Defense attorneys are jumping on this trying to make DNA not be the nail in the coffin for their clients. Prosecutors have been lazily overstating the uniqueness of a 9-point match. The FBI, rather than just acknowledge that a higher match level might be necessary to ensure uniqueness, is seeking court orders to stop wide match searching in its database. This to me seems retarded from the FBI. Wouldn’t you rather crawl the database once, find all of your close matches, then resolve those cases (a few hundred out of 65,000+) so that you can be aware that any of the people involved in those matches will require 11 or 12 point matches if they are on trial. The FBI should just crawl their own database (Google iFBI !) once, and publish the numerical results to DA’s offices nation-wide.
It would seem that you’d want to eliminate the uncertainties that you can, so they don’t bite you in the butt unexpectedly.
The complaints about tying up the database or violating the right to privacy are ludicrous. My laptop could do billions of comparisons in a day. Depending on how hard a comparison is, this shouldn’t take more than overnight, unless the FBI database is running on a TI-85 graphing calculator. Borrow time on a DoE supercomputer overnight and get it done. Doing numerical compilation of the results while havingthe names stripped off the numbers would be sufficient to not violate someone’s privacy. Yes, the whole DNA strand is mostly unique (twins being the outliers), and the profile is apparently less, but still significantly unique. But the counts of comparisons between profiles aren’t unique. It’s analogous to comparing the names of the people in the database and returning the amount of matches among the letters of the names. The names might be private, but the match numbers won’t be.
The 12 Angry Men Blog 
July 21, 2008 at 1:25 pm
(1) One of the dirty little secrets of DNA testing is that the risk of lab error, e.g., the tech mixing up the swabs or mislabeling the evidence envelope, is much larger than that of the computer being wrong. Thus the “one in a bazillion” odds being quoted by attorneys are, as one might suspect, wrong. This doesn’t undermine DNA as a powerful tool but does as an infallible one. No measurement system is perfect. Unfortunately:
(2) Most people massively underestimate the degree to which measurement error is present. A famous study by constitutional lawyer Lawrence Tribe—who happened to have been trained as a mathematician before going into law—showed that the implied false positive and false negative rates deemed acceptable to most lay people would require measurement that’s on par with very high quality blood tests. (The study was cast in terms of utility scaling of false convictions and letting the guilty go but it amounts to a study of the measurement characteristics of the system.) Sadly the jury system handles the HARD cases where there’s ambiguous evidence. Given the way the system works, if it’s at all open and shut one way or the other, it doesn’t hit a jury.
(3) In our adversarial system of justice there is no one in the court system who’s job it is to represent finding out the truth. Both prosecution and defense are attempting to skew the facts one way or the other. This, of course, introduces MORE error in the process.
(4) Frozen raspberries and blueberries are an awesome way to cool down on a hot day! (Sorry, I just can’t help sharing that fact.)
July 21, 2008 at 1:42 pm
(1) Yes, lab error. I knew a place that never paid its lab bills, so it kept getting refused business by any reputable lab. They ended up losing months of tests when their crappy lab that was stupid enough to keep working with them got buried under a wall of water in New Orleans…
So when they say the flood waters were contaminated, there’s nothing like a bio-hazard lab leaching out into the community.
(4) Indeed. The Mrs. Angry Immigrant has replaced all of her ice cube usage with frozen strawberries.
July 21, 2008 at 11:32 pm
Oh, FYI:
http://www.fjc.gov/public/home.nsf/autoframe?openform&url_l=/public/home.nsf/inavgeneral?openpage&url_r=/public/home.nsf/pages/610
This is the official guidelines distributed to judges on how to interpret scientific evidence of various kinds. I don’t know most of the authors but the ones I do know of (Freedman, Petroski) are serious scholars in their fields (statistics and engineering, respectively).