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From: nather@ut-sally.UUCP
Newsgroups: sci.bio,sci.astro,sci.misc
Subject: Re: Mass extinctions
Message-ID: <7697@ut-sally.UUCP>
Date: Thu, 2-Apr-87 10:38:38 EST
Article-I.D.: ut-sally.7697
Posted: Thu Apr  2 10:38:38 1987
Date-Received: Sat, 4-Apr-87 12:53:37 EST
References: <784@scicom.AlphaCDC.COM> <10@slu70.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 28
Xref: utgpu sci.bio:212 sci.astro:791 sci.misc:237

In article <10@slu70.UUCP>, guy@slu70.UUCP (Guy M. Smith) writes:
> Note that the periodicity proposed by Raup and Sepkowski is still rather
> controversial and is disputed by many scientists, especially paleontologists.
> 
> Your friendly net geophysicist,
> 
>                                    Guy M. Smith

...and anyone else who deals with time-series analysis of noisy data.  When
astronomers get results as uncertain, they go back to the telescope to get
more data.  The basic problem lies in the tranformation of noise (as well as
signal) in the power spectrum analysis process.  The resulting noise is far
from friendly (Gaussian) and is, I'm told, a Chi-squared distribution with
n-1 degrees of freedom, where n is the number of data points in the spectrum.
I *do* know it is VERY easy to get "peaks" in the power spectrum due to
happenstance noise buildup that have no basis in reality (whatever *that* is).
This is not to say the claimed periodicity is not real --- just not proven.
I'd suggest more data, with less noise.

How to do that is left as an exercise for the geophysicists ...

Your friendly curbstone astronomer,

-- 
Ed Nather
Astronomy Dept, U of Texas @ Austin
{allegra,ihnp4}!{noao,ut-sally}!utastro!nather
nather@astro.AS.UTEXAS.EDU