GENEALOGY-DNA-L Archives

Archiver > GENEALOGY-DNA > 2002-11 > 1036231805

From: Jack Martin <>
Subject: Re: [DNA] Forensic markers
Date: Sat, 02 Nov 2002 05:10:05 -0500
References: <3f.14433314.2af3fe53@aol.com>

Defense lawyers often attack the appropriateness of the database used to
obtain match statistics, either in terms of the number of samples it
contains (not enough) or in terms of the geographical distribution, or other
issues concerning the participants. I'm not aware of forensic labs that
combine reference databases to do their statistics. Many use the FBI's
database, or one that contains samples from their particular state. As long
as the database has been found to be in Hardy-Weinberg equilibrium and
passes all the other standard statistical tests for validity, it can be used
for its intended purpose.

----- Original Message -----
From: <>
To: <>
Sent: Friday, November 01, 2002 10:57 AM
Subject: Re: [DNA] Forensic markers

> In a message dated 11/01/02 3:13:39 AM Pacific Standard Time,
> writes:
>
> > On the excellent Blackett calculations web page, Mr. B is unique
> > 'probably' in +/- 7,700,000,000,000,000 people (based on 200
> > samples); yet his double may occur - probably - in +/- 500,000
> > samples.
>
> No, it's not HIS double, but SOME double -- when you look at all the
possible
> pairwise comparisons in the database of 500,000 samples (and actually it
> would take a bigger database for the above example). I very belatedly
> received John Chandler's message about the calculations (I saw a partial
> quote in one of your messages before I received the original), and he
> mentions the "birthday problem." This is a famous problem in statistics,
> because the results are so counter-intuitive (a synomym for "you can win
bar
> bets with this"). If you assemble a group of 23 people, there's a 50-50
> chance that SOME pair will have the same birthday, but that does not mean
> that there's a 50-50 chance that YOU will have the same birthday as
someone
> in that group.
>
> I Googled on the phrase "birthday problem" and found numerous sites which
> illustrate it in some detail. Here's just one -- there are others with
> applets to let you calculate different numbers, but don't try them with
big
> numbers! The possible combinations would overwhelm the calculator. In
fact, I
> didn't even know how to set up the calculations that John did on the 1
chance
> in 50,000,000 example without running into trouble on the computer, but
I'll
> trust John on this. The URL may be split by the time it passes through the
> RootsWeb remailer -- it ends with shtml.
>
> http://www.studyworksonline.com/cda/content/explorations/
> 0,,NAV2-76_SEP949,00.shtml
>
> It's been very instructive to look at some of the web sites you provided.
I
> didn't realize that the reference databases had such a small number of
> samples. Of course, there are many sets of small samples, and presumably
they
> are combined where appropriate. I imagine the lawyers argue a lot about
which
> is the suitable reference database for their clients.
>
> Ann Turner
> http://lists.rootsweb.com/index/other/Miscellaneous/GENEALOGY-DNA.html
> DNA preservation kits: http://www.dnafiler.com
>
>
>
>
>
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go to:
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