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Archiver > GENEALOGY-DNA > 2002-11 > 1036166227


From:
Subject: Re: [DNA] Forensic markers
Date: Fri, 1 Nov 2002 10:57:07 EST


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
GENEALOGY-DNA List Administrator
http://lists.rootsweb.com/index/other/Miscellaneous/GENEALOGY-DNA.html
DNA preservation kits: http://www.dnafiler.com




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