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Archiver > GENEALOGY-DNA > 2003-11 > 1068916982


From:
Subject: Re: [DNA] Time to Most Recent Common Ancestor
Date: Sat, 15 Nov 2003 12:23:02 EST


In a message dated 11/14/03 2:50:49 PM Pacific Standard Time,
writes:

> I would appreciate some assistance understanding the probability of time
> to most recent common ancestor. The material FTDNA sent me with my 25-marker
> YDNA results includes some graphs of probability vs. Time to MRCA. If I
> eyeball those probability distributions I note that the mode of the 24/25 curve
> is about 7 generations and the median or 50% point is roughly 11 generations.
> Yet the table included with the FTDNA material says that the 50%
> probability point in that MRCA distribution is no longer than 18 generations.
> Similarly, eyeballing the 23/25 graph indicates that the mode is about 14
> generations and the median or 50% point is roughly 18 generations. But Ann
> Turner's MRCA calculator gives 28 generations for the median of this
> probability distribution.
> Can someone help me understand this?

I see what you mean, but the appearances are kind of deceptive. The curves
are very "skewed" (a statistical term for situations where the data is not
symmetrically arranged around the center). The curves extend a long way to the
right of center, so that pulls the median to the right, too.

Bruce Walsh has written some more background about time to the MRCA. There
are some cumulative probability graphs on those web pages that help you read off
where the 50% number actually falls.

http://nitro.biosci.arizona.edu/ftdna/TMRCA.html

Ann Turner - GENEALOGY-DNA List Administrator
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