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Archiver > GENEALOGY-DNA > 2010-11 > 1290457913


From: (John Chandler)
Subject: Re: [DNA] P value (was chances are, it's wrong)
Date: Mon, 22 Nov 2010 15:31:53 -0500
References: <F9C440A2-FC59-4A9E-AAAC-85DEE9D2FAB0@GMAIL.COM>, ,<COL115-W50D879F102DC3996D9D454A03A0@phx.gbl>, ,<4CE7A3C0.7050702@ucl.ac.uk>, ,<COL115-W1464B78AF0292D6AEFA183A03B0@phx.gbl>,<COL115-W5950BB2C58A31B4806036EA03B0@phx.gbl>,<4CE8088B.3020500@ucl.ac.uk>,<COL115-W45724B549DCDA5DD2EC4A0A03B0@phx.gbl><COL115-W424C7732D1583F8960685CA03B0@phx.gbl><4CE851AF.2030304@ucl.ac.uk>
In-Reply-To: <4CE851AF.2030304@ucl.ac.uk> (message from James Heald on Sat, 20Nov 2010 22:54:39 +0000)


James wrote:
> Suppose the upper confidence limit is 50 generations. That means that
> if the TMRCA actually was 50 generations, it would produce n or fewer
> mutations 5% of the time.

You are mistaking confidence in the outcome for confidence in the
estimate. The 95% confidence interval for an estimated parameter
is the range which includes 95% of the cumulative probability
distribution for that parameter. This somewhat arbitrary interval
is popular because it happens to coincide very nearly with the
"plus or minus two standard deviations" range of a Gaussian. In
terms of percentiles, the interval runs from 2.5 to 97.5, meaning
that the likelihood of exceeding the upper limit is only 2.5%, not
5%, and the likelihood of falling below the lower limit is the other
2.5%.

John Chandler


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